Hello My 12 week old has started rolling during tummy time (tummy to back but no sign of trying to roll back to tummy). He already sleeps with both arms out, I was wondering if he needs to now transition to a crib rather than the bassinet?

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

I think it would be safest to also transfer him to his cot now.
Seems like he is not far off rolling during his sleep time
Red Nose recommends - space of the cot & arms free, & firm, flat mattress to provide safe environment once rolling.
Remember, still always place him on his back for sleep.

Increase his tummy time when awake - more frequently if he is not tolerating long periods on his tummy.
Is he holding his head up &/or reaching for toys?
Increased tummy time when awake helps to build upper body strength to assist with rolling both ways.
Continue with nothing loose, soft or padded in his cot.