Mini-ITX is a small form factor motherboard that uses smaller and less expensive components than full-size motherboards. Many cases and PCs designed for mini-ITX use fewer expansion slots, so it's not uncommon to find a mini-ITX motherboard that doesn't fit a 3090. If the case or PC has room for an add-in card, the mini-ITX board can usually accommodate it. However, if you need to route cables through the exterior of the case, you'll likely have to get a larger mini-ITX board.