Bed wobbling on Ender 3

If the V roller wheels aren't tight on the Y axis beam, it means the eccentric nuts are not adjusted correctly. Two of the rollers are mounted centered on the holes in the carriage frame, but the other two are on eccentric nuts which displace them from center slightly depending on the orientation the nut is turned to, to allow tightening and loosening of the grip on the beam. Since the Y axis ones are hard to see under the bed, look at the X or Z ones to get an idea what to expect.

Note that the bolt through the whole roller assembly can loosen when adjusting the eccentric nut. You can probably avoid this by figuring out the right direction to turn it and only going that direction (continuing around just under 360 degrees if you go too far). If you do loosen the bolt then the eccentric nut will move on its own under vibration, so you need to re-tighten it. For the Y axis this might require taking off the bed or taking the carriage off the beam (by removing the belt and tensioner).