Hello my baby is 3 months old and sleeps in a cot in my room. She has started to roll into her side only in her cot. I have her in an arms out sleeping bag now since I saw this. She can't roll over yet. Should I be moving her back to her back when she does this? I'm worried I won't notice her do this during the night when I'm asleep.

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 her new environment as safe as possible.
She needs to be “arms out” & in a cot which you have already done.
She may be close to rolling to her tummy, so she needs her arms free & the space of her cot to roll safely.
Ensure that the cot mattress is firm & flat & there is nothing soft or loose in her cot.

Have you tried turning her back onto her back? Does she stay there?

How does she go with tummy time when awake?
Increase tummy time to help build up her upper body strength.
Always continue to place baby on back to sleep.

Red Nose has the following general information now that she is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over