I have a 2003 Civic Hybrid.
It's overheating.
I replaced the radiator,
And the water pump,
And the head gasket.
I'm out of ideas.

 

What about the thermostat and fan switch?

 

You may have an air bubble in the system.
Replace the hoses, they may be collapsing.
Use a premium thermostat, one that fails in the open position.

 

When does it overheat? in stop and go traffic? while going up steep grades? while sitting and idling?

I bet it's air in the system or an improper coolant/distilled water mix causing it. There is a slight possibility you have a crack in your head or block which acts as a great nucleating spot for steam that results in air and steam in the system this often causes newer cars to overheat… it might not even leak yet on a newer car. I would only suspect this if it had been driven while overheated quite a bit.

Also possible is that it's running too lean, this can cause your car to heat up fast, have you heard it make any unusual sounds like heavy pinging or knocking up long grades.

And as the above poster mentioned make sure you have a premium thermostat in your car, it really doesn't cost much more and it makes a world of a difference if it gets stuck.