Success! I have all three camera's calibrated! I had to mess around with the calibration script quite a bit to override the error bounds for the projection errors between narrow FOV centre and very wide left and right, seems to work though!
Also, I love how our black cat, Toby, is literally a void in the depth map! 😅
Thank you to everyone for your help, I'll post my code up once I've got it all checked in.