Besides trying what Dick mentioned (and yes, 3 old injectors CAN cause a rough idle IF they're dirty), there is a way to run a can a pressurized injector flush through your injectors with the proper fittings. I havent yet needed to do it on an 850 but we do it on occasion with RWD Volvos using Motorcraft injector flush (sold at any Ford dealer, maybe parts stotres?) and the stuff works wonders on those cars (rough idle, lack of power complaint cars).
Kent Moore tools sells the fittings for the older RWD cars so I'd imagine that they'ld supply something for an 850 type fuel system as well. A dealers parts department that's willing to work with on that could probably call Kent Moore tools and look into it if desired.

Dicks idea is by far the cheapest, easiest way to start however.

As for dealers, we too have that problem and it's too bad that because the new lube kid botched your oil change, you've decided that everyone else there is equally incompetant. The dealers problem (any shops really) is that when they need more help, they hire the first kid who walks in the door and that guy sometimes has problems just doing oil changes. If you ever go back to a dealer, you might try requesting that their Master Tech works on your car and yes, you can request that.