Albion had won all four of their previous home games this season without conceding a goal.
But that impressive record was ripped to shreds by a four-goal Cardiff blitz in the space of 30 first-half minutes.
Fowler, 32, helped himself to a double while Hasselbaink, 35, and Sinclair, 34, also got in on the act with two smart pieces of finishing.
Albion manager Tony Mowbray, who had warned his players about the threat of Fowler and Hasselbaink before the game, made seven changes to the side that had won their previous game at Scunthorpe and the re-shaped team couldn't cope with Cardiff's key trio.
Ishmael Miller briefly gave Albion hope of what would have been a remarkable comeback in the 33rd minute, but Mowbray's team couldn't add to that strike before the on-loan Manchester City marksman struck again from the spot three minutes from time.
Fowler, who had opened his Cardiff account with a double against Preston last weekend, took just three minutes to set City on the way to an impressive victory .
Albion's defence let a Stephen McPhail cross flash across the face of goal and Fowler latched on to the ball before beating Dean Kiely with a rising drive from 12 yards.
Craig Beattie and Miller both wasted good chances to equalise before three goals in just seven minutes sealed Cardiff's victory.
Albion central defenders Martin Albrechtsen and Shelton Martis made the mistake of backing away from Hasselbaink and the Dutchman took full advantage of the space he was given to score from 20 yards in the 23rd minute.
Hasselbaink then turned creator four minutes later by releasing Joe Ledley whose run into the area was halted by a clumsy challenge by Martis.
Referee Peter Walton had no hesitation in pointing to the spot and Fowler helped himself to his second goal by sending Kiely the wrong way from the spot.
Sinclair then got in on the act after 29 minutes - rifling home a cross from Tony Capaldi that Albion had again allowed to run across the face of goal.
The only positive Albion could take from the game was that Miller continued his impressive run of form.
He pounced with a low drive from 12 yards in the 33rd minute after Beattie's initial shot had cannoned off the post.
Cardiff could afford to ease back in the second half and Albion predictably had more of the play but an heroic comeback proved to be beyond them.
They did at least carry a threat with Chris Brunt seeing his 55th minute free-kick fly into the side netting and Albrechtsen having a 61st minute header was cleared off the line by Sinclair.
Substitute Sherjill MacDonald was also denied a goal by a brave 72nd minute block by Roger Johnson, while Miller had a drive deflected off the line by Capaldi.
Miller was eventually rewarded for his hard work when he was upended by Chris Gunter and converted from the spot - taking his goal tally to four in six games.