Hi nmcampbell and welcome to CoffeeSnobs.
Unless the commercial machine is relatively new they wont sell into the commercial market..... so your target audience is home use....
Now as you have discovered a 3 group is huge (even my 2 group is big) and these suffer severely from lack of WAF (wife approval factor) ::) ::) They also require plumbing and a high current (20 Amps or more for a 3 group) special power run. These are expensive to instal and, although there are work arounds, this requirement will deter most potential buyers.
So whilst a single group commercial might fetch over $1000. a two group (even in good condition) can be struggling to get the mid $500s...... a 3 group and above will be even harder to sell and fetch less.
Id basically put it up for sale with no reserve and just take your chance.... but dont expect much unless it is really fairly new (and it doesnt sound like it is) or is a very collectable model which has been rebuilt - and it doesnt fit that either. Might have to write this off to experience (or hope someone else will snap up the "bargain")
Best of luck.