The fixing screws are there, not only to hold the motherboard in place, but also to reduce any flexing force. Notice the four screw positions around the removable card slots, they help brace the MB when you install a card. I'm surprised there are only three screws, rather than four, around the memory slots, as those are the slots that are accessed most by upgraders. Personally, I'd like to see another one between the SATA connectors and the main power socket, just to steady that corner.
When there is, I keep a supply of plastic "push-through" mounts, which rest on the case and clip through the hole in the motherboard. This at least holds the corner of the board steady when pushing (RAM for example) into the slots.