Wing weight isn't a part of the load on the wing's spar in flight. The wings in flight actually have a negative weight- they weigh less than zero as far as the spar is concerned. All of the stress on a spar comes from fuselage weight. That's even true on a flying wing design as the center of the wing carrying fuel, engine, batteries, etc. is the only load the wings see.
That said, 3 G is not much of a safety margin for a model. Sure, a gently flown scale plane might not ever see more than that, but aerobatics planes routinely see 10 G or more. I think if it were mine (just as opinion here) I'd start with 5 times the fuselage weight and hold the wings at the middle when testing. That doesn't perfectly replicate flight loads, but it would tell you if the design is sound. Others with more knowledge and experience than me may weigh in with better suggestions.