U3 refers to a spin-cycle interruption in documented AQUA models. Check how the laundry is distributed and the support; if the manual covers it, also check drainage.
The official AQUA FAQ links the warning to vibration or load and, in certain sections, to draining conditions.
A small load can also become unbalanced
Wait for the drum to stop and the lid to unlock. Untangle the clothes and check whether the weight is concentrated in a laundry bag or a single bulky item. Do not keep trying with a waterproof textile the machine does not accept, even if the rest of the laundry is compatible.
Adjust the load to the program without overfilling it to make up for one garment. Check the stability of the support with the appliance stopped; a tilt in the machine is not corrected by redistributing the laundry.
If water remains and the manual links U3 to restricted outflow, follow that external check. Do not force the spin cycle to a higher speed. Repetition with normal loads and correct installation requires service.
Report whether U3 appears only with a specific item or also with a mixed load; that difference helps evaluate usage.