When there has been a sale for the $3 character slot in the past, is the sale limited to 1 at the $3 price, or could I potentially get more than that? I was going to buy some more slots and wait for a sale but if it's limited to just one, I probably will just get what I want for now at the normal price.

Usually these are limited to one, but i think I remember that once it was limited to 5. But generally, you should expect that it is limited

Thanks! 

Yeah, each time I buy one, it's sold out afterwards.  
Pretty much means it's limited to one.