To be honest I've always been curious about out a threesome but couldn't find 2 males that harmonize enough to share without killing each other. But that's ok - I'd feel quite uncomfortable too, if I had to share a handsome guy with another woman.
How is your plant set up? I know you folks do it a little differently across the pond.
Depends on what type of plant you are interested in.

I am not sure how many plants I am supervising atm. Around 60 I believe. The range goes from simple not aerated lagoons to constructed wetland, trickling filters, rotating biological contactors, all the way up to what I believe you call activated sludge process reactors (basicaly a huge swimming pool with oxygen supply where bacteria eat the pollutants out of the free water, or off the flocs, rather). All types with secondary treatment (settling the floc), many even with tertiary (one with UV-treatment).
Funnily, I have no membrane reactors nor sand filters at all in my district - I guess that's because we have very cold winters; they freeze too easily.
The smallest one is for about 20 BOD PE and the biggest is around 125,000 BOD PE if memory serves (I'd have to look up the precise data at work).
If you tell me what type you're interested in, I'll PM you the exact specifications.
Most of the time I don't get much involved, though. Usually I am just called in for help if something goes wrong. For example: now that the pears are ripe, some facilities near juice factories will have trouble with swimming sludge (not sure if that's the correct term - I just translated the German term literally). Pears contain lots of pectines which make the sludge as stiff as beaten egg whites.
The only chance you have in such a case is to switch off the aeration and run the plant anaerobic. (The filamentous bacteria make a net and catch the air bubbles. Normally, they don't stick together but with the pectines they do). Additionally, you can weigh the sludge down with iron salts but we try to avoid that because they'd have to be filtered out again which is rather expensive.