It is certainly possible to combine the three functions in one machine. And there are others out there selling low priced TIG units, anyway. TIG is all about control and cheaper units give up a lot of bells, and also have undertainties relative to service cost, in fact that's a problem with inverters in general.
When it comes to plasma, yo need an air supply, built in units are often described as unreliable, and externals normally are much higher priced. There are consumables with plasma, and you would want to be sure you can use off the rack stuff. No idea about the capcity for plasma of a samll invergter. The average welder pumps out a lot less heat than what is required to burn through plate. Also, anything nice like tool steel or chromoly is normally better cut with something that does not require so much grinding back. Great for making parts out of mild steel though. Never used this type of plasma.