Mr Cameron said he had come through the challenging television head-to-head debates successfully, stressing: “We are winning the argument on the economy and we have got some momentum now to go into these last few days.”
However, polls suggest there has been no extra boost for the Tories.
A weekly ICM poll shows the party unchanged on 33%, Labour unchanged on 28% and the LibDems down two points on 28%.
And a daily YouGov snapshot places the Conservatives unchanged on 34%, the LibDems up one on 29% and Labour unchanged on 28%.
LibDems’ leader Nick Clegg, meanwhile, claimed Gordon Brown had become desperate after the Labour leader launched a personal attack on Mr Clegg, likening him to a game show host.
The Prime Minister also branded the Tory manifesto a horror show. Mr Brown insisted he was in a “fight to the finish” and rounded on the media for concentrating on “froth” over policy.
Earlier, David Cameron acknowledged that, if elected, his government would have to go further than he had said so far in terms of spending cuts.
Today, the three leaders will take part in a fourth public head-to-head in London, organised by the faith-based group Citizens UK.
The election battle in Scotland also reached fever pitch after Scottish party leaders angrily clashed during a heated live TV debate.
First Minister Alex Salmond, Scottish Secretary Jim Murphy, shadow Scottish secretary David Mundell and LibDems’ Scottish spokesman Alistair Carmichael were involved in angry exchanges over issues that included flipping homes, outgoing MPs’ pay-offs, tax and Trident in BBC Scotland’s Big Debate.
In the debate in the Festival Theatre in Edinburgh, BBC presenter Glenn Campbell accused Mr Murphy of “flipping” his homes.
He was asked: “Why did you flip your second home from London to East Renfrewshire?”.
But that led to a warning from Mr Murphy. He said: “These are very serious allegations you’re making. I didn’t do that sort of thing.
“Some politicians flipped their homes and tried to make profits. When I became Secretary of State for Scotland, (I) spent more time in Scotland and as a consequence that becomes (my) first home.”
The practice of “flipping”, where MPs were accused of changing the designation of their homes for financial gain, was criticised during the Westminster expenses scandal.
Mr Murphy changed the designation of his first home from London to his house in East Renfrewshire after he became Scottish Secretary.
But Labour insisted he had done this to comply with the rules.
Mr Murphy said: “As Secretary of State for Scotland I spend much more time in Scotland, so East Renfrewshire is my first home.”
The four participants clashed over issues including budget cuts, tax, immigration and the Trident nuclear weapons system and the presenter struggled to control the debate.
Mr Salmond, who is not standing for Westminster, was attacked by Mr Mundell for claiming the MP’s redundancy pay-off even though he is not leaving politics.
He said: “His £65,000 Golden Goodbye is not automatic. Commons rules say he has to apply for it.
“It is a tax free redundancy payment for MPs leaving politics. Alex Salmond, as he admits, is not leaving politics. He will carry on getting more than £100,000 salary from the taxpayer, as well as a Westminster pension.
“He is taking the taxpayer for a ride by taking this extra cash under false pretences.”
The SNP leader donates money from his MSP’s salary to a charitable trust and said he would also donate at least £10,000 of his resettlement grant to help good causes in the north-east of Scotland.
The Tories and SNP also clashed over Trident, which the SNP claimed should be cut instead of public services.
Mr Carmichael, meanwhile, asked Mr Murphy if he was “embarrassed” that the richest people still pay a lower proportion of their income on tax than the poorest.
He said: “People on low and middle incomes are giving a subsidy to the super rich and that’s wrong”.
“We’re not going to change that overnight but one thing we could do is take the first £10,000 of everybody’s income out of taxation altogether. That would make a real difference for
people on low and middle incomes, it would be £700 a year in the pocket of every taxpayer in Scotland, and that would be paid for by closing the tax loopholes at the top.”
Today, all four of the main parties have events taking place across Scotland.
SNP depute leader Nicola Sturgeon was on a tour of seats in the west, with a visit to Paisley, while Alex Salmond began a whistlestop tour of other key seats in Tayside, Aberdeenshire, and Central Scotland.
Meanwhile, Mr Murphy was unveiling Labour’s latest election poster in Edinburgh.
The LibDems’ deputy leader Vince Cable was meeting business leaders and students, as well as taking part in a question and answer session, in Aberdeen.
And Scottish Tories’ leader Annabel Goldie and Gary Mulvaney, the candidate for Argyll and Bute, were unveiling the Tories latest “celebrity endorsement”.
How the campaign has influenced local voters
Andy Colvin, 45, community worker, Dunlop
Which leader has impressed you most during the campaign?
All three had set answers, I was disappointed.
What issues are most important to you?
Savage cuts and people who will lose their jobs.
Have the TV debates persuaded you to change your mind?
Nick Clegg has swayed me and I never considered the LibDems before. We have the SNP in Scotland, but I feel sorry for the rest of the UK.
Who will you be voting for?
None.
Geraldine McGeechan, 25, student, Prestwick
Which leader has impressed you most during the campaign?
None, they are trying to be the same, telling us what we want to hear.
What issues are most important to you?
Education, jobs and the common people.
Have the TV debates persuaded you to change your mind?
Didn’t watch them.
Who will you be voting for?
Probably Labour.
I think Gordon Brown has been quite substantial and down to earth.
Kenneth Elliot, 41, sales director, Glasgow
Which leader has impressed you most during the campaign?
Nick Clegg, get rid of what’s in. I feel the public’s trust has been compromised with the expenses scandal.
What issues are most important to you?
Getting more help for small businesses -- it is ridiculous banks are rejecting loan requests.
Have the TV debates persuaded you to change your mind?
I’m debating whether to even go to the polls.
Who will you be voting for?
Undecided.
Jim Hughes, 61, retired, Cumbernauld
Which leader has impressed you most during the campaign?
Probably Nick Clegg -- he comes across more honest than the other two.
What issues are most important to you? Improvements in the health service.
Have the TV debates persuaded you to change your mind?
The debates have been interesting and worthwhile and they could sway my vote.
Who will you be voting for?
Conservative
Laura Paterson, 19, student, Glasgow
Which leader has impressed you most during the campaign?
Gordon Brown, he just seems a bit more down to earth than the other two.
What issues are most important to you?
Education and health.
Have the TV debates persuaded you to change your mind?
No -- have not watched any of the debates.
Who will you be voting for?
99% sure it will be Labour.
