I have a very slightly different setup, two "in front" and one in "Center back". I believe you can update this fairly easily.
Relevant statements extracted from config.g:
; Driver to axis mapping
M584 X0 Y1 U2 Z3:4:5 ; X and Y for CoreXY. U for toolchanger lock. Z has three drivers for kinematic bed suspension.
M569 P3 S0 ; Drive 3 direction | Front Left Z
M569 P4 S0 ; Drive 4 direction | Front Right Z
M569 P5 S0 ; Drive 5 direction | Back Z
; Leadscrew locations (really, kinematic coupling locations) extracted from CAD model assuming back right build plate corner is (0, 0)
M671 X300:5:152.5 Y316:316:-14 S10 ; Front Left: (300, 316) | Front Right: (5, 316) | Back: (-14, 152.5)
M350 Z16 I1 ; Set 16x microstepping for axes. Use interpolation.
M906 X2500 Y2500 Z1000 E1250 I30 ; Motor currents (mA) and Idle percentage
M201 X600 Y600 Z10 E1000 U1000 ; Accelerations (mm/s^2)
M203 X25000 Y25000 Z400 E4000 U10000 ; Maximum speeds (mm/min)
M566 X700 Y700 Z2 E3000 U200 ; Maximum jerk speeds mm/minute
M92 Z3200 ; Steps/mm for Z - TR8*4 / 0.9 deg stepper
And bed.g (note that front left is a little offset in Y because of a temporary thing that blocks that corner of the bed):
G0 X152.5 Y40 H1 F10000
G30 P0 X152.5 Y40 Z-99999 ; probe near back leadscrew
G0 X275 Y260 H1 F10000
G30 P1 X275 Y260 Z-99999 ; probe near front left leadscrew
G0 X25 Y315 H1 F10000
G30 P2 X25 Y315 Z-99999 S3 ; probe near front right leadscrew and calibrate 3 motors
G1 X0 Y0 F10000