We have a 3 week old who rolls onto her sides to sleep. We reposition her onto her back very often but she just keeps rolling back. Being 3 weeks old, we have not observed her rolling during her very brief movement awake and the side sleeping is obviously concerning for us. Any suggestions?

Most babies do not roll intentionally before 3 months; however, some babies do roll, especially to the side, at a very early age. This is not unusual.

Are you wrapping her?
Some babies do not respond well to wrapping/swaddling & prefer to have their arms free.         
Some babies like to touch their face or suck fingers when settling.
You could try settling her without any swaddle, or wrap with her arms out. (as for 3 mth old)

https://rednose.org.au/article/wrapping-babies

Are you using blankets?
Is she in a bassinet or cot?

Have you commenced tummy time yet when awake?
https://rednose.org.au/article/tummy-time-is-good-for-baby-because
Always place baby on the back for sleep.

Red Nose does not recommend the use of sleep positioners.
Red Nose does not recommend positional products such as anti-roll devices and items that fasten a baby into a sleeping position.
Products that restrict the movement of a baby or a baby’s head should not be used.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position