I'm not sure if this is what you wanted but here it is
www.gamemaker3d.com/player?pid=051530018846
There are 3 'If Key Pressed' conditions I have placed inside the Player's 'Loop'.
If you press keyboard number 1 Player will have full visibility (visible:1, alpha:1)
If you press keyboard number 2 Player will have full invisibility (visible:0)
If you press keyboard number 3 Player will have partial visibility (visible:1, alpha:0.5)
I hope this is what you had in mind.