If you are just mining,  it's generally more economical to use risers and 5-7 cards per board. Also, X99 is pointless, it costs a ton more than AM3 or 115X platforms and also uses more power. If you don't want the hassle of obtaining or building a custom enclosure, then a 4 physical x16 board like biostar racing is a better bet than using 3 cards. If you get cards with good coolers, like MSI twin frozr then you dont necessarily need a slot gap between each card (I wouldn't recommend pre Ellesmere AMD cards next to each other though).