The key to sharpening, be it a broadhead or a knife, is raising a burr and grinding it off.
With the 3 blade heads, you're going to be doing two edges at once. Lay your file on on the broadhead (or the broadhead on the file) so it's flush with 2 blades. File away till you can feel a burr on the opposite side of the two blades, It doesn't need to be a HUGE burr, as long as your fingernail catches it, you're good. At this point, rotate the head 120 degrees and start the burr raising process again.
What you want to do is get a burr on all 6 sides of the cutting surface, and then grind it off. With the 3 blade heads, as soon as you rotate it, you're going to be grinding off one of the burrs you just raised. Pay attention and make sure that you do get 6 burrs raised.
Once this is done, make a series of passes on each side with lighter pressure till there's no burr left on any of the blades. I like to start this out with say 5 passes per side, then 4, then 3, ect ect; rotating the head so each side gets the same number of passes; 5 on one side, five on the second side, five on the third side.
At this point, you could use a stone and repeat the last bit (the 5, 4, 3....bit). This will polish up the edge and the head should be seriously sharp.
So, the Cliffs notes version: get 6 burrs, grind them all off, have no burrs left at the end, and then polish it a bit.