if you REALLY want to power a laser and a spindle from M3, why don't you have the M3 run via another output which will control which device is in use.
Bear with me......
Say you have a spindle that needs a digital input to power it.
feed the M3 for the spindle via a NO relay which is powered from another output, then either run M7/M8 or even any other macro number you choose to toggle that spindle interlock.
say you use M500 as your spindle interlock and M501 to toggle the interlock off.
So if you want the spindle to run, you would run.
M500
M3 Sxxxxx
.....
M5
M501
now you want the laser....
M3
M10 Qxxxx
.....
M11
M5