I can tell you with 100% assurance that one Rx 480 GB will work fine on an old HP board with DDR2 Ram and should be the same on your ASUS board. You can also try with 2 cards and that is the max I would try on such board with that amount of Ram and lga 775 (as an IT I can tell you that lga stand for land grid array which I learned in CompTIA certification and is the way Intel numbers their processors) with 775 being one of the slowest Intel release so going with three cards in your motherboard is a lost cause in my opinion.

But if you can borrow a third card just to try , you can do it, you can always experiment, as experimenting costs nothing.