Originally Posted by demise

3 years starts counting from the day your aos is approved, regardless of if you get a conditional (2 yr) or unconditional (10 yr) green card. Only limitation is that your n-400 cannot be processed until your i-751 is approved.

So lets say this happens:
You get your conditional green card.
21 months pass you and you file i-751.
Uscis drags their feet on your i-751. You can actually file n-400 up to 90 days prior to your third anniversary of aos even if your i-751 is still pending.