There is a slight wobble from left to right maybe 3mm. Is that normal for a ruckus

 

What are you running, the stock wheel or a custom wheel with a hub?
The stock wheel could be bent. Does it wobble at speed or when not moving? (or both?)
If you have a fatty wheel, (DWT polished aluminum) then it is normal because its two wheels welded together and not balanced properly.
Check the axle nut and/or lug nuts are tightened. Maybe try Dyna-Beads to balance it.

 

 

Ok I did some checking, and yes it is a common problem.
It could be the wheel bearings need replacing, a bent axle(I hope not), or the washer is missing for the axle nut.
The stock wheels are not the best quality steel and have a tendency to warp after falls or potholes.
Take the wheel off and check for axle wobble. Check the wheel bearings for damage. Double check that axle nut & washer are tight.
Hope this helps.