Competitive Siblings (how to prevent babies rolling over at night)

The girls are now 3 months old. They have been very good to their mummy, having slept all night since a months ago!!! I really cannot complain.
Then Gwen started rolling over a week ago. I think this is quite fast, considering Eddie only started rolling over at 6 months. It is a nightmare because when they roll onto their stomachs in the middle of the night, they get stuck and can't roll back. Of course they scream for your attention to help them roll back. What I used to do was put bolsters under his mattress, one on each side. This created a valley which he slept in and he couldn't roll over due to the slopes. With the twins, I substituted the bolsters with their supply of wet wipes.
This afternoon, Lillyth rolled over. Speaking of sibling competitiion!
My mozilla is sick therefore I can't upload photos for the time being.