The tourists, who had been a solid 85 for one at lunch, lost four wickets for 79 runs in the second session, with Durham paceman Onions's figures now standing at an economical two for 32 in 18 overs.
Fellow seamer Tim Bresnan weighed in with two for 44 in 16.
Marlon Samuels, who scored a century in England's nine-wicket second Test win at Trent Bridge, was 50 not out and Denesh Ramdin one not out.
History was against either side winning this match after the first two days had been washed out without a ball bowled.
Only twice before in Test history had a team triumphed in such circumstances, with England beating New Zealand at Leeds in 1958 and New Zealand defeating Bangladesh at Hamilton in 2001.
It was no surprise when England captain Andrew Strauss opted to field first after winning the toss against a West Indies side whose top-order collapses had helped the hosts into an unassailable 2-0 lead in this three-Test series.
Before this match, England had left James Anderson out of their 12-man squad in order to rest the Lancashire seamer and, before play started Saturday, they also omitted Stuart Broad.
Onions and Steven Finn were called up in place of the new-ball duo and, in overcast conditions, it was Onions, in his first Test in over two years, who nearly had an early breakthrough.
Adrian Barath had made just four when Onions took his outside edge only for Ian Bell, fielding in the third slip position often occupied by Anderson, to drop the straightforward chance.
Although he had looked the least threatening of England's three seamers, it was Bresnan who took the first wicket when he had Kieran Powell caught by second slip Graeme Swann for 24 to leave the tourists 49 for one.
However, it wasn't long before debutant Assad Fudadin, in for the the dropped Kirk Edwards, drove Finn through the covers for four.
Off-spinner Swann came on in a change to England's all right-arm pace attack but Barath responded by advancing down the pitch to loft him for a straight six.
Barath, 40 not out at lunch, had yet to add to his interval score when he was reprieved again by third slip Bell, who dropped an easy chance after Finn took the shoulder of the bat.
Fortunately for England, Barath had added just one run when he was lbw to an understandably delighted Onions, his exit only temporarily delayed by an unsuccessful review, as a 106-ball innings, featuring a six and four fours, came to an end.
And 90 for two soon became 99 for three when Finn caught and bowled Darren Bravo after the batsman checked a drive.
Fudadin, who went more than half an hour without scoring, saw his promising innings end on 28 when he gloved a well-directed Bresnan bouncer and Bell, to the delight of his Warwickshire home crowd, at last held a catch.
Samuels then drove Bresnan through the covers before striking successive Swann deliveries for a six and a four to complete a 76-ball fifty.
But shortly before tea Narsingh Deonarine, in for the injured Shivnarine Chanderpaul, edged Onions to Strauss at first slip
