I'd like to touch on something I ranted about a bit in the April 19 Improbable Insights podcast. The full-tower case is a dinosaur.
Look, I know some of you out there love your triple-GPU, overclocked, liquid-cooled monster PCs. I love that you love building and using these lumbering beasts, and more power to you. However, most people don't game on triple-4K displays, and the headaches of managing SLI and CrossFire to get a good gaming experience gives me heartburn thinking about it. I know, because I've run SLI rigs, only to be disappointed with lackluster game support, awful image artifacts, and all that heat. I suppose it's a good thing that DX12 offers improved support for multiple GPUs, but game publishers still see multi-GPU setups as fringe cases. (Haha, see what I did there?)
Unless you're dead set on running three GPUs, you don't need a full-size ATX motherboard. Most higher-end micro-ATX boards implement SLI and CrossFireX support, so you can run your twin graphics cards if you so desire. Micro-ATX mobos typically have four expansion slots; with the right slot setup, you could have your dual GPUs plus another card, be it a PCIe SSD or sound card. You can find a rich selection of micro-ATX motherboards offering serious overclocking support, amenities for liquid cooling, and other high-end features. Only a few years ago, only a few paltry micro ATX boards existed, mainly serving price-conscious buyers. Not so today.
Mini-ITX motherboards allow you to build even smaller systems, as I did with my itty-bitty gaming rig. As with micro-ATX boards, the selection of mini-ITX boards expanded substantially over the past few years, and even include boards aimed at high end gaming — though you're still limited to one graphics card.