I agree with M&S... if you provide enough "levels" they could be fine... it is usual to sat that 10 gallons per gecko is enough, but I have always thaught that since leos don't have adhesive lamellae, they don't need so much room, meaning, a 10 gallon tank can be more flat, having more space for the gecko, or more high, having less space for the gecko.... if your 20 gallon is not so tall, or you can provide more space dividing it into levels in my personal opinion 3 geckos could do fine, of course, the more space the better, but they can make it there