The firms were interviewed by the trust's project manager Fuller Peiser in early September and a winner should be announced by the end of this month.
A source close the trust said: "The line-up may come as something of a surprise to designers in Britain. Although all three architects have excellent reputations for designing laboratory buildings, NBBJ is not well known in the UK."
NBBJ – Naramore, Bain, Brady and Johanson – employs around 800 designers and has offices across the USA, with four international offices in London, Tokyo, Oslo and Taipei.
The line-up may come as a surprise to designers in Britain – NBBJ is not well known in the UK
Source close to the Wellcome Trust
The Wellcome Trust refused to comment on the shortlist, but confirmed that a winner will be chosen within a fortnight. A spokesperson for the trust said it was currently preparing a revised planning application for the project to replace the submission that was rejected by South Cambridgeshire council last September.
The trust's proposal to extend the genome campus, designed by Sheppard Robson, has already been the subject of two public inquiries. In 1999 the government ruled against the trust's original proposal of a 40,000 m2 extension. A source said: "The trust is very sensitive about this project because it is so close to Hinxton, which is a small village where residents are concerned about losing greenfield land."
One of the shortlisted firms said it was thrilled just to have been picked.
