The ATX motherboards have up to 7 PCI-E slots and 4 Memory slots. They use a 24 pin power connector. They allow you to connect up to 4 RAMs up to the given maximum memory limit. They also tend to have more SATA ports and front-panel header ports like USB 3.0, HDMI and many other ports.
The Micro ATX motherboards support up to 4 PCI-E slots and up to 4 RAM slots. They also feature a 24-pin power connector.
The Mini ITX motherboards have only one PCI-E slot and 2 RAM slots. Most of them will come with inbuilt hardware for wireless and Bluetooth connectivity. They use a 24-pin connector.
The ATX Motherboards will fit into ATX cases. The Micro ATX motherboards will fit into both ATX and Micro ATX cases. The Mini ITX motherboards will fit into ATX cases, Micro ATX cases and Mini ITX cases.
In general, the motherboards fit into cases that are of their size and those that are larger than them. So, a Mini ITX motherboard will fit into all the above 3 cases but an ATX board will not fit only in a case designed for Micro ATX or a Mini ITX board.
The ATX motherboards are the clear winner in this category. Thanks to those 7 PCI-E slots, you can run a multi-GPU setup without any problems. You can also connect various expansion cards like sound cards, network modems, Bluetooth modems and so on. This type of motherboard helps effective cable management and offers better cooling compared to the other cards.
Micro ATX is right in the middle of the other two form factors. Most of the time 4 expansion slots will be enough for many PC builders. You also won’t be having any problems with RAM management.
The Mini ITX, on the other hand, has only one PCI-E slot. Clearly, these Mini ITX motherboards are not for gamers. They also consume less power among all the three types. Most of the graphic cards will have problems fitting into the Mini ITX case. You only have to work with 2 RAM slots. There won’t be any proper cooling facility in the Mini ITX cases.