Multiply the length and width together to get the total required area - 5*8 = 40. Now each tile covers 3 inches and you need to cover 40 feet, so convert feet into inches by multiplying 40 by 12 (480). Now divide 480 (desired area) by 3 (how much each tile covers) to get 160 - the amount of tiles you'll need.
Sorry, didn't realize I didn't post it as an answer. But no problem!
