Can I use 8ft cedar pickets and 2 x 4 rails (3 rows) on steel posts that were originally placed for a 6ft. ?

The answer is going to depend on the size of the existing steel posts and the depth they're in the ground. Also if they're supported by concrete or just the ground. If the steel posts are from a residential chain link post they're both too small a diameter and too far apart to support an 8' tall fence. An 8' high privacy fence should have posts not more than 8' apart and in the ground deep enough to be below the frost line for your area. The three rail idea is perfect for an 8' tall fence. The problem really comes down to the wind load on a fence this high. Wet ground and a thunder storm could blow the fence over if the posts are too small, too far apart, or not deep enough.