Please clarify. You purchased R720 with a 2.5" HD backplane but you made an order of 3.5" HD. We do not have adapters for use of 3.5" HDs in the 2.5" HD backplane. However, there are adapters for use of 2.5" HDs in the 3.5" HD backplane.
If this is the case, the only option available is to order for the 2.5" HDs.
Let me know if you have any further questions.