Does this require any wiring harness or relay, or does the switch handle everything?

The switch is included, but confirm whether a separate relay harness is needed for your vehicle—most fog setups require one. Your Civic's service manual will specify the wiring path.

Will these mount directly to the factory fog light holes, or do I need brackets?

Test-fit before committing. Factory mounting points vary; if the housings don't align with existing holes, you may need adapter brackets or minor fitment work.

Can I swap in a different bulb type later, or am I locked into H3?

H3 is the socket type for these lights. You can upgrade to a higher-output H3 bulb, but the socket itself won't accept other bulb types like H7 or 9005.