IMO, if you ordered and paid for 3mg, but the vendor sent you 6mg...I'd contact them. Request a bottle of either the Grape eliquid in 0mg so you can cut the 6mg with it (without diluting the flavor)...or a bottle in 3mg that you should have gotten originally. It was their error, and they should make it right.
While waiting for 1 of those, why not just use the 6mg at the lower watts that you said weren't so harsh?