For KSAN (Lindbergh Field), the best spot is Laurel Street. There is a small parking lot at the corner of Laurel and Harbor Drive right next to the airport perimeter fence, and a larger one across Harbor. You may have some difficulty getting photos from there though, as there are a couple of fences in the way (if you have access to a truck or SUV that will help). Alternatively, you can walk towards Pacific Highway along Laurel, and take photos from the sidewalk. There is also a parking lot right next to you, but it is privately owned by Solar Turbine and they're not very spotter friendly. Some security guy may eventually come by and ask you to leave.
KMYF (Montgomery Field) has an open ramp. You can pretty much drive anywhere, get out and take photos. If you see someone, it's a good idea to make sure they don't mind if you take photos of their airplane just to be courteous.
KSDM (Brown Field) is pretty much the same as KMYF.
Hope this helps,
David / SAN