you must set your MSC to autorize the usage of A5/3 in the BSS. Other than that, no other problem.
Also, to strengthen the A5/1 algo, you can activate the “LapDm filling with random bits” feature. It make sthe A5/1 much harder to crack !
(reason : not all MS are A5/3 compatible, so even if you activate A5/3, you’ll stil get a lot of A5/1 in your bss)