So let's be honest ... Mixing up  2oz of three different buttercream drives me insane - but not as bad as figuring out how to plate it nicely with out having to use three piping bags that I will have to wash or dispose of! How do the professionals do it on CC? 

Why mix up only 2oz of each... Couldn't you make and freeze a batch?  Or just use what flavors you are currently using?

As for piping... you could always use a 2oz scoop and just place a pretty scoop on the plate of each...

You could even use the scoop to pre-scoop out little scoops and freeze in that shape...

I am under the presumption you serve the three buttercreams as a mix-n-match tasting with cake slices in this situation... 

Freezing is more work - SMBC tastes better when it's been whipped after its comes out the freezer  

Just curious to see what others are doing  

SMBC is the only frosting I use and I freeze mine all the time. You are right, it needs to be rewhipped. What works for me, I freeze them in "scoop" amounts and will pull them out a couple of hours prior and take a spoon and rewhip. That still taste amazing

I'd say make a batch,  pipe pretty rosettes, and freeze till needed, but you say it needs to be re-whipped. SO I'll tell you the easiest way to wash your piping bags: squirt dish soap directly into the bag, rub between your hands ON THE DRY OUTSIDE ON THE BAG, then rinse  out. As for the piping tube, drop in a cup with a squirt of dish soap and water to cover. Microwave 30 secs. Remove cup, swish, pour out, rinse.