The four X and Y axles are locked in place by the pulleys. However, it is possible that one or more of the pulleys can become slightly loose over time, which could affect their alignment. If this is the case, there could be play on the X and/or Y axles which can cause issues with print quality.
It is recommended to check for play on the axles at least once every three months.
Start with the right X axle. Place the print head in the back-left corner of the printer to keep it out of the way. Hold the frame of the Ultimaker 3 with one hand, and firmly hold the right X axle with the other. Attempt to move the axle forwards and backwards; do not be afraid to apply too much force.
Repeat this for the other axles. Make sure to move the print head to the opposite side each time.
The axles should not move at all. If one of the axles does move, you can observe a ticking noise of the pulleys hitting the frame. In this case it is recommended to calibrate the print head.