It will just be too narrow. There is maybe a very slim chance that a rear wheel with deep offset could rub - but i very much doubt this. Jaco Prism has deepest offset i think, but i think if it is ok on 12X with no spacers it should be ok on G3 with no spacers.

Yes the standard steering block for G3 is the non ballraced std block. The EU edition 12X uses the ballraced version with axles and bits to fit US wheels. With all the parts it would fit G3.