The most comfortable sports bra I have found is the Champion Double Dry Distance bra. It's not their most supportive (I'm also a 34D and it's not quite enough for running), but for cycling I think it would be good. I usually wear a different bra by them for running, but it's super compressive and not as comfy. If you're not running you probably don't need as much support.

I'm not sure how it would work for a tri. You could probably swim in it, though it is double-layered and I don't know how that would work for the swim portion. I am sure it wouldn't dry as fast as a tri top, but those aren't nearly as supportive, either.