I've been a Riven player for quite a while, and I thought this, too, when I first started.
For her early game it is just terrible. It just is not viable. Though once you have a core of 3 items or so, building a Tri-Force is viable.
Frankly, you need to get some survivability or any CC will reveal your squishiness. I usually like a Frozen Mallet or Warmogs. Then I usually build some Damage like a BloodThirster. Then its all situational. TriForce really helps double your damage later in the game, it is a good 4th/5th item.