Stand by switch?

I have a Marshall MG 100FX head.love it,question is ,how feasable to have a three way switch and maybe a cooling fan installed in it,3-way= on ,off, stand by cooling fan. They come with a simple on-off push button ,or would it be a waste of time and money?
Thanks ,
David

The amp is solid state, so it would end up being best to use a "Mute" switch/circuit - perhaps at the input jack.

As far as a fan, is heat really that bad?

If so, you'll need to figure out where you can fit the fan, how to power it (likely a 120v AC fan and tap into the AC input circuit), and then we can figure out how to put it on the 3-way Power/Mute/Fan switch... if its possible do to in a practical way, that is.

If it was a tube amp, I'd know of more than a few ways to do this kind of thing, but since I can't even find a schematic for the MG series amps, I'm not sure if its possible.

You'll need a separate switch for muting (shorting the input) A DPDT would do the trick if you have 2 inputs.
As for the fan, consider a thermal switch mounted in thermal contact on the power transistors' heatsink. That way if the amp overheats for some reason, the fan can come on automatically.
These are made by Klixxon (you'd need a Normally Open--N.O. type) but you'll also need to figure out at what heatsink temperature you want it to snap (and you want a switch designed for that temperature).
You can measure the heatsink temperature with a probe after the amp has been used for a good while (with an input source and at high volume) and a Digital Multimeter and select a trip point perhaps 20% above that.

Alternatively, find a quiet (brushless) muffin fan and let it run at all times.

This having been said, I am not at all convinced a fan is necessary, though.

SS amps have no need for a standby switch... just turn it off when not playing, and turn it back on when you want to play it.