At the time I am without my studio monitors and my only interface with a mono switch. Is there a mono switch for the master buss to check my mix in x3?

You can make a bus or main out mono by toggling its Mono/Stereo switch, any "upstream" meters, that is, meters on any tracks or buses that are assigned to the mono bus/main out-from the bus/main out, will also display as mono.

Note: this only affects the meter display, not any audio that is bounced or exported.