The figure emerged as the Conservatives pushed ahead with plans for a like-for-like system, announcing multi- million pound contracts for the design of new submarines.
The SNP condemned the proposals for what it said would be "immoral, outdated and grotesquely expensive" weapons.
The Liberal Democrats oppose a like-for-like replacement and the two parties have agreed a stalemate where no final decision will be made until after the 2015 general election.
But the Conservative Defence Secretary Philip Hammond admitted that billions of pounds of spending will already have been committed by that stage.
Speaking in the House of Commons, he said: "Some £3 billion has been earmarked for spending before the next election."
His comments came as he announced £79 million of investment in the next generation of submarines which will carry the nuclear deterrent. Mr Hammond said it was essential the orders were made now to ensure the submarines would be ready in time.
Angus Robertson, SNP leader at Westminster and the party's defence spokesperson, said: "The determination to spend £100 billion on Trident nuclear weapons that are designed to flatten Russia is perverse."