xbx 360 RROD? have tried a hot air gun reflow + x-clamp replacement?

ok so i bought some xbox's with the intention of fixing and selling them. they all have 3 red rings. i have tried using a hot air gun to reflow the board but it hasn't worked. i also added in bolts and plastic/ metal washers to all of them but still nothing. one of them turns on then beeps and goes to 3 red lights. another one goes on for about 20 seconds (with no picture or sound) then goes to 3 red lights. the other one just has 3 red lights. i was wondering what else i can do? thanks