Been mining for about a month now, just about to finish my first dedicated openair-rig with three 280x and my old 6870, handbuilt.
Keeping two 280x's busy with Worldcoins atm and playing around/learning with the rest. Gonna try solo mining for the first time tonight with Giftcoin.
Will probably expand the rig with another 280x within a week or two.
What power supply are you using? I have a 1300W EVGA that I ended up picking for the 10 year warranty, and it seems like it could handle 3 290X cards right now.