how do i store a 3 tier buttercream cake that i made today and not due until tommorrow night? i dont want it to dry out but to big for me to cover.

Hi, has it been frosted? If so, you can leave it out a few days or so with buttercream or fondant. I've left them out two days (because the surprise person missed their plane and they person picking it up asked if they could wait an extra day to bring it home) and they raved about the cake.
HTH

Sure - it will be fine sitting out for a few days - As long as you dont have any parishable filling inside and its not 100 dgr. in your house - it will be fine =}

if i leave buttercream frosted cake out of refrigerator and not covered in any way -- will the frosting become dry?

It will be just fine. Even many of the big bakeries do it.