Hi, We put our just turned 3 month old baby down in her bassinet on her back. She wears a sleeping bag. During the night she manages to wiggle around to a 90° angle or sometimes we find her head where her feet were when we put her to bed. How do we stop her from doing this? Thanks

Many babies will wriggle & squirm in the bassinet when they are trying to learn to roll, especially by 3 mths of age.
Some babies like to touch their face or suck on their fingers.
Most babies will be rolling by 4-5mths….. some earlier, some later.
Wriggling &/or rolling does not need to be stopped - it is a normal development stage.
is important to continue to provide a safe sleep environment.

Most babies who are wriggling like this, at this age, & when in a bassinet & swaddled, should be moved to a cot, as they may be at risk of getting stuck in an unsafe position if/when trying to roll.
Babies need space of a cot & arms free to roll well.
Always continue to place baby on back for sleep.
Is the sleeping bag one with arms out or in (like a swaddle)?

Tummy time is very important for building upper body muscles to assist with rolling ability.
How does she go with tummy time when awake?
Rolling is not something that needs to be stopped but continue to make her sleep environment as safe as is possible.
Have a read of this article: https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over