I have explained the difference between a 3 gang switch and a 3 way switch. I have also mentioned that a 3 gang switch is actually 3 individual 1 gang switch bundled side by side so as to save space and to make multiple switches lined up on the wall look neater. As such, terminating the wire terminals in a 3 gang switch as very similar to terminating a one gang switch.
Refer to the diagram above. If you look at the back portion of a 3 gang switch, you will actually see the 3 individual switches. As mentioned before, the Live terminal will normally be indicated by the words L1 and the switch wires indicated by L2. It might differ with different manufacturers but it works the same way.
The live wire from the Mains supplies electricity to L1 in the first switch. In order for the second and third switch to have this same supply, we simply connect a short strand of wire from the fist switch to the second and third switch. This way, all the three switches now will have electrical supply.
A 3 gang switch naturally means you have 3 lights to control, so there must be 3 switch wires or wires that goes to the individual lights. Just terminate those three wires separately into each switch.
It is also suffice to mention that if however you mixed up the wires, or terminated them upside down, it will not cause ant major problems like an explosion or anything. The worst you might faced might be a light that cannot be switched off OR a switch that works in the opposite direction. In any case, a small relocation of the wires for the affected switch should take care of the problem.
Like always, make sure to switch OFF the Mains before attempting to terminate any switches or electrical outlets.