Hi, I have a 3 month and one week old who for the first time rolled into her stomach in the cot. She did it again later in the night and was face down but not crying out. I have read that their brains have not yet developed to alert them to breathing issues. Is there anything I can do to make it safer for her to sleep? I am worried she’ll roll and I won’t hear it to reposition.

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.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Have you transferred her to a cot now & ceased swaddling?
Babies need their arms free & the space of a cot to roll well.

Is the cot mattress firm & flat?
Is there any loose or soft objects in her cot?
How does she go with tummy time when awake? Is she holding her head well &/or reaching for toys?
Does she roll/wriggle around the cot (or floor) when awake?
What does she do if you roll her back or turn her face o the side when you see her on her side or tummy or face planted?

Always still place her on her back for sleep.

Have a read of the Red Nose 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”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over