If you wanna use Triforce, you're gonna be using an entirely poke based playstyle. Take Tlords.
What you wanna do is Q+W+auto+dash out for trading. It'll do decent damage and you won't take much damage.
Your all in will be pretty weak compared to normal Riven, unless you're 1) against a tanky champion who will take more than a normal fast combo and 2) you have enough AD for the attackspeed/Sheen proc to outdamage AD stats, and 3) If you're replacing cleaver, you'll also have to make up for no armor shred.
It's not a terrible item, per se, but it's too expensive and has too much gold in stats that Riven doesn't use (or rarely uses), like the mana and attack speed. Compared to most other items, it's pretty shit. Especially if you're using the same amount of gold.