it depends on how much traffic will be carried by the inner zone. For 3 TRX outer, you can plan at least 2 inner trx, and up to 4.
The point is to unload the outer zone, because the outer zone frequencies are more difficult to plan. If you remove 1 outer trx, it means one free frequency. By putting 4 inner trx, you might be able to remove 1 outer trx.
I hope I was clear :))