Simple. Run the correct size wire first. (Warning) On a 15 amp circuit run 14/2 wire and on a 20 amp circuit run 12/2 wire. Now run power to that 3 gang light box and make sure to leave at least 1 foot of wire sticking out. You will have 4 wire groups in the box. One power, and three wires to the lights. Label the power wire so you do not get them confused or just strip that one black power wire back 3/4 inch. Take all the white wires cut them to about 6", strip them back 3/4 "and tie then all together under a wire nut and push them back into the back of the box. Now take all the ground wires and tie them together plus a jumper ground wire about 12" long. Push them back in the box and use that ground jumper wire to connect to all the ground screws on the 3 switches. Jump from one switch to another. Make sure the switches are orientated correctly in the up/on, down/off position. Keep it well clear of the other screws on the switch. Now take the black power wire connect and connect to one switch and jump it from one switch to another connecting it too all the bottom or top screws, makes no difference. Now take the black wires going to the 3 lights and connect them to the screw left on each 3 switches. Determine which switch you want to be in which location. Put everything back in the box making sure all screws are very tight, and the wires are secured behind the screw and no ground wire is touching the power or light screws on the switches. This is how to wire single pole switches. 3 Way switches are different.
Take the always hot wire and attach 3 six inch pieces (some call them pigtails) of the same size wire as the source by using a large wire nut. Then attach the other end of the each pigtail to each of the 3 switches. DON"T FORGET TO SHUT OFF POWER.