I'll provide some numbers, starting with Capela's first season as a regular starter.
Clint Capela 2016-17 stats against Milwaukee - 2 games, but 1 where he only played 10 minutes:
10.5PPG - 3RPG - 1APG
2016-17 stats v New Orleans - 4 games:
9.8PPG - 9RPG - 0.5APG
2017-18 vs Milwaukee - 1 game:
10PPG - 8RPG - 0APG
2017-18 vs New Orleans - 4 games:
16.8PPG - 11.3RPG - 0.3APG
2018-19 vs Milwaukee - 2 games:
17.5PPG - 12RPG - 1.5APG
2018-19 vs New Orleans - 3 games:
7.3PPG - 10.7RPG - 1APG
This is all a pretty basic look at how his stats stack up when he comes up against Davis and Giannis.
Anthony Davis for the most part experiences very little statistical downturn when he comes up against Houston except for this year, but this year is a bit of an anomaly with AD. His shooting percentages are relatively normal, usually hovering around 50%, and he tends to score around 25 on them with 10 RPG.
Giannis on the other hand routinely dominates Houston. In 2016-17 he averaged 31.5PPG - 9RPG - 4.5APG against them. In 2017-18 he averaged 29PPG - 8RPG - 3APG against them, and in 2018-19 he averaged 23PPG - 17.5RPG - 4.5APG against them.
So from this very basic look at their performances against one another we can see that while Capela has gotten better at getting second chance points and feeding off Harden, he struggles to stop either of these two from scoring or rebounding. I'm sure someone can do a better analysis than me, but I don't think there's much of a need to go any deeper than their elementary stats.
I personally don't think it's worth trading for him. I like Capela, but I think he really benefits from playing next to Harden, and his passing makes me worried about how he'd fit into Brad Stevens' offence.