T3 Crystals? The 25 AP or 50 AP ones would be the best, I think. Also, Ultimates in the current Union event (about to end in a few hours) drop them in droves. But RNG is going to RNG, so you might not see them often if you're unlucky, from any source.

T3 Crystals are way, WAY more common than T4 though, so if you've been getting T4, then you're lucky... as odd as that is.