Have you tried a new TCIU? Also try plugging an external monitor directly into the front of the TCIU and see what sort of image you get.
We don't look after any -300's at the mo but from what I remember you can isolate each camera with a CB on the front of the TCIU to see if any one camera is affecting the image of all 3.
Just some ideas it might be worth trying.
p.s. The external monitor will need to have an NTSC input, the front fitting is BNC.