Technically, you don't need *any* limit switches.
You could use the printer just fine with manual homing - you move the axes to what you want as their origins (i.e. to their limit + a little margin), then issue a G92 command to set that as origin.
The only two reasons 3 limit switches are there is:
1) to automate the homing
2) for ease of mind if you have them set up to always halt the movement if hit (even during printing)
Considering how much time people spend adjusting their bed level, 1) is of dubious importance, as you'll end up spending time on homing one way or the other (at some point). 2) is irrelevant, as it only helps if the gcode somehow tries a move beyond the limits and the work area width/height/depth in firmware somehow gets corrupted (if that happens, you've got more problems to worry about than hitting the limit).
Draw your own conclusions as to what use 6 limit switches are