Tri-suited doesn't really make us want to 3bet less. If anything, our flush component is less necessary in a HU pot than a MW pot. We also don't mind 3b/folding as much with tri-suit.
In general we hate flatting SB, when there's a limper, although most don't run sims with limpers, it stands to reason that we want to 3b more and not less with hands that want to iso.
If you feel that your hand is not strong enough to 3b, you should just fold.
Against a standard btn range, 3b and flat EV are relatively similar (-1084 vs -841), but with the added incentive of isolating, and also generally wanting a higher 3b% in these games, I think a 3bet is justified.
I will point out however, vs co range, which could also be a reasonable estimation of btn's iso range, we lose quite a lot by 3betting. In this scenario, we fold sb, but EV is similar, and I wouldn't hate flatting. Again due to dynamics of a softer game, I also wouldn't hate a 3b as an explo to widen ranges, but clearly there's a great EV difference if we're against a CO frequency.
That said, the issue with going MW is still present, so I believe that will vastly reduce our calling EV, so the real answer is to just fold.
Also, this is 200 rake, so at 50 rake, I think flatting just goes entirely out of the picture, so it's just a clear muck if you don't think the iso is light enough.