Spatch - Nobody seems to have asked what turns your plasma on, and what delay you need - or - indeed, what system you are using.
If you are using the output from a PC - via your printer port, then Mach 3 can put out on/off signals on various pins to turn thngs on and off.
The standard printer port is three ports in one. Pins 2 to 9 are an eight pin output, Pins 1,14,16 and 17 are outputs and Pins 10 - 13, and 15 are inputs. If you are using a different interface, such as a smooth stepper, these will differ, but you should have the documentation for that.
Is is normal (but not necessary) to use the outputs 2 - 9 to control the axis - each axis requiring 2 wires. If you are only using 6 of the wires, you can use the other two for something else. We can consider than you might have pins 8,9,1,14,16 and 17 spare as outputs. These can be allocated on Mach 3 to various functions.
If you go into Config/Ports and Pins/Spindle Setup you will see, on the left hand column reference to M3,M4,M7 and M8. These can be indirectly allocated to pins, so that when GCode M3 is used, the pin for M3 goes high (or low) and similarly for all the others. To turn off M3 and M4, an M5 command is used To turn off M7 and M8 an M9 command is used. These outputs are allocated to outputs #1 to #6.
If you now turn to Config/Ports and Pins/Output signals your outputs can be allocated to pin numbers on your output port.
The signal from this output pin should then drive a relay, which in turn can drive whatever application you want. I have four outputs, all driving 5 volt relays via a Darlington array. From these relays I drive my spindle inverter, and (if I ever get it going) my coolant system.
The use of the M3, M4, M7 and M8 are not compulsory, there are other ways of driving outputs, but they are well know Gcodes - you are merely using then for a different purpose to that intended.
If you can identify from you GCode what signal you CAM program is putting out to turn on your plasma, then you can configure the appropriate output on Mach 3, and wire a relay into the system to turn your plasma on. Bear in mind it is a different Code to turn off.
I don't know if this is what you were wanting.