How much ram you got in there? I would suggest 12gb for 3 290's to be sure its not the issue. 4gigs will NOT work, you might get away w 8gb.  Also, I'm assuming you reinstalled the latest ati drivers after plugging em all in? Also, I forget if 290's have dual bios switch's, maybe u flipped one by mistake?
Do all 3 cards work independently? It will take some time but check all cards one at a time, then check all slots. Hope you get em running, ive had similar issues & its fu*king annoying esp when u've spent mad cash on em!