The notion of the B3 having an overloaded structure is not an issue.
If I recall my studies correctly, the 2250 Kg internal MTOW of the B2 and B3 isn't a structural limitation but is rather a control limit for "the average pilot sans hydraulics". The B3 has a more robust MGB than the B2 and a different build spec for the MRB's. The hook is different too, plus lots of other bits. The structure itself is just fine at the B3's external MTOW (plus a bit).
It's kinda like saying the 407 is just a jazzed-up 206 so be careful not to exceed the 206's loads. The 407 does have the odd difference to its predecessor and it also seems to be handling its tasks OK. Not that I'd want one myself though.