If you bought from a vendor which has stock and you have proof that your RMA has been approved (along with proof of a wait of "3 days or 3 months") then you might be able to arrange for a direct replacement through them.

3 days seems reasonable to me. But if you seriously expect to wait up to 3 months (and you've had the product for less than 30 days) then simply return it, take the restock penalty on the chin, and order another mobo which is in stock. I always buy computer gear local when I can, online only when I must, so that I always have a direct product return option when necessary - without shipping costs/delays and other hassles. "I am not satisfied with this product" is really the only reason I need to give.