Take it back and ask the shop to see whats wrong. Don't ask them what they did wrong.
They should deal with the diagnosis with an appropriate point of view if you don't come at them aggressively.
With that said don't allow much of what they find to be your problem. If they are fair they will probably unerwrite much of their diagnostics costs even if there is another problem.
Remember from their point of view the current problem is no easier than the first problem (maybe a whole lot harder).
Your description rings no bells in my head but from a lot of experience I would replace the sparkplug connectors under the three coils. They often cause load related misfiring.
Bosch Master, ASE Master, L1
33 years MB technician