David Cameron warned of the "massive risk" of putting Labour in charge of the economy, while Ed Miliband said the future of the NHS was the "big choice" on 7 May.
Meanwhile, there were scuffles at a Labour campaign event in Scotland.
And the Lib Dems and Conservatives clashed over claims David Cameron had admitted he would not win the election.
Lord Scriven suggested the prime minister had made the admission in a private conversation with the Lib Dem leader Nick Clegg - deputy PM - six weeks ago.
The Tories said the claim was "100% not true", and David Cameron told party activists that victory was "within our grasp".
In other election news:
Opinion polls currently suggest no party will win an outright victory and another hung Parliament is likely, despite both the Conservatives and Labour claiming they could still seize victory on their own.
In a message on Twitter, the Lib Dem campaign spokesman Lord Scriven said he had been told by his party leader that Mr Cameron admitted to him in a private conversation that the Tories wouldn't win a majority.
Speaking on the BBC's Victoria Derbyshire show, Lord Scriven -  a former leader of Sheffield City Council - said he would not "take back anything" he had said and defended his decision to speak out at this late stage in the campaign.
"When I read... David Cameron is not telling the truth to try to scare people to vote then I think his private fears do need to be made clear," he said.
Mr Clegg declined to comment on the detail of Lord Scriven's claim, but said it was a "big, fat fib" to suggest the Tories could win the 323 seats effectively needed to command a majority in the House of Commons.
"They are not going to get 323 seats and they know it," he said on a visit to south-west London.
In response, a Conservative spokesman said the claim was "100% not true".
But a senior Liberal Democrat source said: "I can categorically confirm that the prime minister did say the words as described by Lord Scriven in a conversation with the deputy prime minister."
Mr Cameron said Mr Clegg was "increasingly desperate".
Speaking at a campaign rally in Bath, the PM said: "We've got three days left to go to make these arguments and it all comes down to one thing, it all comes down to one idea, and that is about the economy.
"When you are in the voting booth, with that stubby pencil in your hand, ask yourself this question: do I trust Ed Miliband to run the British economy?"
Mr Miliband, who hailed cookery writer Delia Smith's endorsement of his party at a rally in Hove, said the NHS was "fighting for its life because of the choices this government has made".
He called for a report by former Marks and Spencer boss Stuart Rose on NHS reform to be published, adding: "The future of the NHS is at risk in a way it hasn't been in a generation."
Mr Miliband also said the row with the Lib Dems "says it all" about Mr Cameron's campaign. "Other leaders, I gather, are conceding the outcome of this election.... We are fighting every step of the way."
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Meanwhile, scuffles broke out on the streets of Glasgow when Jim Murphy, Labour's leader in Scotland, tried to address activists in the city centre.
Protestors playing loud music and shouting "Red Tories out", drowned out the politician as he tried to make a speech alongside the comedian Eddie Izzard.
Mr Murphy said it was evidence that the SNP were trying to disrupt the democratic process. But demonstrators interviewed by the BBC denied they were there on behalf of the SNP.
Amid speculation about what will happen in the event of an inconclusive result, Plaid Cymru leader Leanne Wood said she would be "staggered" if Labour believed it could automatically count on their support and that of other parties in the event of an inconclusive result.
"If Labour want our support to run a government effectively they need to take on board some of the things that we are saying. It's arrogant of them to just assume that they can just take our votes without giving anything back in return," she told Today.
She added: "We would be prepared to vote down a Budget by Labour if it was pushing, putting forward, more cuts on the backs of the poor."
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Party leaders have set out rival versions of the "choice" facing voters three days before the general election.