Unfortuanllty mate its how main dealers work.
I had a constant missfire on my car and i complained loads and i mean loads (TFSI Mike knows what i have been through)
Dealers have a set procedure to investigate issues , and will not deviate from it.
even though your injector probably is the issue , i would think that because they are paying the labour , they are probably trying to look at parts they can remove and try easily before getting into the time comsuming things like removing your intake manifold to replace your injector.
Incidently missfires and injector issues on cylinder 3 are very common.
By the way my issue turned out to be the timing being incorrectly set for over 3 years , even though i was told that my car wouldnt even run if the timing was out.
There is a lot to be gained from reading these forums and learning from peoples experiences , and a good independant specialist is worth 10 main dealers any time !
good luck mate keep us posted.