As noted above you might want to reduce the main fuse to 60A to protect the relay, or go with a higher capacity relay.
It's not a problem to run all three of the switches off of a single fuse since the relay coils don't draw much current. This is a pretty standard setup. I'd recommend using crimp connectors instead of solder since soldered connections can fail due to the vibrations in automotive use. If you really want to solder the connections then be sure to support the wire on both sides of the joint so it can't flex and work harden.
Personally, I like to have most of my accessories on constant (non-ignition switched) power. This way I can take the keys with me when using the air compressor to air up the tires, or if I want to have lights on when setting up camp. If I have to leave the key in the ignition I worry about accidentally locking it in the car. It also prevents the car from constantly making a dinging sound to warn that the key is in the ignition.