My bitch was C-sectioned a little over three weeks ago. There was no discharge after the C-section until about four days ago and now there is a fair amount of bloody discharge. It is dark brown and not bright and she is not running a temp and seems fine otherwise.
I just got home and there is really a bit of a mess where she was lying on the couch and of course, it is 11 at night.
Is this common? Any input would be good while I wait for the vet to open in the morning.

Dark brown is old blood, not active bleeding. After whelping, this is called lochia and it is seen up to 6 weeks after whelping. It represents the processed blood from the placental sites in the uterus.
If she was not spayed during the c-section, it is entirely normal for her to be discharging this brownish old blood, and as long as there is no pus or odor to the discharge, no worries.

My girl had that with her c section... it was old blood though not bright red... I would def call yor vet!

Yes, the only two things that worry me are that it didn't start for almost three weeks after the section and that there is more than in a girl who has whelped naturally.
Otherwise, it looks normal.

Hopefully your girl was put on Clavamox after the C?
If so, sounds normal to me.