My bed is held at three points. I want Marlin to tap at those three points for autobed level. Currently my Marlin firmware is set to tap at four points, creating a rectangle of taps, namely:

// these are the positions on the bed to do the probing
#define LEFT_PROBE_BED_POSITION 15
#define RIGHT_PROBE_BED_POSITION 170
#define BACK_PROBE_BED_POSITION 180
#define FRONT_PROBE_BED_POSITION 20

How do I set Marlin to probe only three points that I define instead of four points? Also, G28 causes the hot end to home in the middle of the bed. How do I get it to home at the front left corner which is where it homed before I implemented auto bed leveling?