This isn't that difficult a mod to perform as many folks just place the switches directly into the spot in their pickguard where the 5-way switch was originally. You'll have to drill the slot out a bit for each (mini)switch, so it isn't reversible without getting a new pickguard.
Position N M B Selected 1 X Neck only 2 X X Neck + Middle 3 X Middle only 4 X X Middle + Bridge 5 X Bridge only New X X Neck + Bridge New X X X Neck + Middle + Bridge
The disadvantage of having three separate switches instead of one, however, is that if, say, you want to switch from the bridge pickup to the neck, you have to flip two separate switches instead of just one. This isn't that big a deal in the scheme of things, but I found I got tired of that when I set up one of my Steinbergers with the three-switch approach.
Additional possibilities are available
if, instead of using the switches as strictly "Off/On," you used DPDT switches
instead to switch pickups into series with one another. In fact,
this is precisely what Brian May(of Queen)'s guitar does (he has three
separate switches dedicated to each pickup... plus phase
switches, but that's a story for another time). The three single
coil pickups are added in series to with one another, a very clever wiring