EV3 turning in circles
It is quite common for the tires to slip and cause this type of behavior. For example, here is a similar question. The answer here is the same.


Make sure your center of gravity puts most of the weight of your bot on the drive wheels.
Make sure the tires and the surface your are driving on are free of dirt/dust.
Use sensors to help compensate for any slippage of the wheels.


If you only want your robot to go in a perfect circle and nothing else, you could fix a beam to the floor on one end an your robot on the other so that it cannot deviate from the circle.