You cna use the initialization function line.
Create a macro that loades the correct Gcode program, such as M2222
'M2222 Macro to autoload program
Loadfile("C:\mach3\Gcode\roadrunner.tap") 'Name of requested Gcode file
End
Then add in the Mcode to the init line in Gen/config.
When Mach3 open and you press the reset button it will LOAD the program.
(;-) TP