If I'm not mistaken, this would be the first time a different party was one of the three largest parties in the Commons since the election of 1918.
Depends on how you deal with the SDP breakaway from Labour. Definitively the SDP were bigger than the Liberals in the early 80s (29 MPs at its peak), and whilst it is a Lib Dem predecessor, that was far from clear in its early days.
If you just restrict it to general election results, you may run into problems in 1931 when the Liberal National Party finished 3rd ahead of the Liberal Party (from whom they had broken away). The LNP eventually became a part of the Conservatives, though it took the better part of 30 years for the merger to go through.
