The max legal width is 296mm. For 4x4 short course it is supposed to be measured at the axle centers but I haven't raced anywhere where that's been the case. Most places measure overall width (which can be affected by camber and toe angles) with a big gauge/cutout, or they place the truck in a box and see if it rolls out.
Either way I don't see the +3s being legal, even if you ran very little camber and toe.
That being said, unless you're running at an actual ROAR event, I wouldn't worry about it, just run the +3s.
Lastly, I'm assuming that all your tires are already glued to these +3 rims