Carry the problem one step further and tell us what happens when you have 2 out of 3 lights on and then flip the first switch.

The two lights go out

To describe more fully: These are 20 amp three-way switches.
Switch 1, with the two wire plus ground wire power supply, is at the garage.
Switch 2, terminating the line, is at the porch

In between Switches 1 and 2 are three dome lights in a row -- call them A, B, and C.

I can throw the garage Switch 1, and lights A,B, and C all come on. If I then walk to the porch and throw Switch 2, lights A,B and C all turn off. If I then walk back to the garage and throw the switch 1, only lights A and C will come on. The middle light will not light. If I then go back to the porch and throw switch 2, A and C go out and B remains out. Go back to the garage and throw switch 1, and all three come on.

I have checked each connection several times to make sure I haven't mixed up travelers and common terminals on the switches, but I expect I'm missing something somewhere along the line.

Forget what you are trying to do, and forget the physical orientation of everything.
Wire between the two switch with 3-wire, DO NOT go to any of the lights with this.
Feed one switch and run the switch leg to the lights from the other switch.

Too many folks think just because the lights are physically in between the switches they can easily wire them that way. It can be done, but you need special wire and a complicated wiring scheme.

What do you mean by 3 wire? 12-3 with ground? And then use what kind of wire to come back to the lights in sequence? 12-2 with ground?

I also suggest that you follow the advice of Petey. The questions in the last post do not make me confident that this person is at all ready to do this installation correctly.
Get a professional on site.

As Stated there are no physical lights in between 3 way switches. Neutral wiring aside one 3 way switch has the hot wire from the panel with 2 traveler wires going between 3 way switches and the other 3 way switch has switch leg to the switched lighting. If this does not make sense you should ask for assistance from technical help before installing.