Is Triforce a good item on Riven?

Not really, no.

There are worse items, but it's better to just build AD and CDR.

Well, the AS is almost entirely useless on her, the Phage is better built as a Black Cleaver which provides things she wants (CDR, more AD, Armor Penetration/Reduction, and the movespeed is on physical damage instead of autoattack), and due to animation cancelling she won't likely get any more than 2-3 Trinity Procs off.

You could build it, but a Black Cleaver and some Lifesteal/more damage will work out better.

Veeeeeerrry situationally I've seen it work, the only issue is that it has a ton of relatively useless stats and that gold could be spent on a much more slot efficient item. the sheen effect and rage passive are nice, and that's about it really. I'd almost always choose more raw AD, armor pen, and CDR over a Trinity force.