By this info, You need a tune-up as technician suggested.
At same time, ask them to read the possible error codes on
sensors and check if injector is leaking and check if air filter
is blocked. It seems you car running on too rich gas mixture.
Why third cylinder not working? Lose presure or simply because the spark plug or
wire. If it loses pressure, then you cannot fix without pay big money. spark plug
or wire is very easy.