Antennas are going to work better when they have a proper ground plane under them. The amount of ground plane you need will matter:
144.430 = about 38" diameter
GMRS = about 12" diameter
CB = about 18' diameter.
Plus, shadowing from the vehicle body.
Easy to put the VHF antenna on the roof, at least 19" in from any edge. That'll give you the best possible performance with an ideal ground plane. Same for GMRS, at least 6" in.
Putting either of those antennas down on the fender is going to result in shadowing that will result in reduced performance to the rear of the vehicle. It will also provide a lopsided ground plane that will make the antenna somewhat directional.
Getting a full 1/4 wave ground plane for CB is impossible on any road legal vehicle. There just isn't any way to get a full ground plane under the antenna. Shadowing isn't much of an issue since the upper part of the vehicle is way less than a 1/4 wave. The CB antenna is going to suffer to some extent no matter where you put it, so mounting it on the fender is not likely to be super noticable. Yeah, ideally you want them all on the center of the roof, but that wasn't one of the options you gave.
The another way to look at this is:
Which radio service is most important to you? If ham or GMRS are not as important as CB, then put the CB in the better location (top of roof).
As for the VHF & UHF antennas, keep in mind that all coaxial cable has some amount of loss. Those losses go up with cable length, the losses also go up with frequency. More cable means less signal to/from your radio. Higher frequencies suffer more. You would gain a slight advantage by using the mount that had the shortest cable run on the roof for GMRS (higher frequency) and put the 2 meter antenna on the other roof mount with more cable (less impact on the lower frequency). But that's kind of splitting hairs.
