The switch will have 3 contacts on it (a common, and then the two "on" feeds)
The middle common connection, you should take this to the relay for the spots (in reply to your second question, YES, definately run the lights through a relay)
The other two "on" terminals go to whatever you want to run it, so you could take one to the blue/white main beam feed (so they come "on" with main beam.... actually, you only need to take one feed, from the blue/white, so the spots will come on whenever main beam is activated, be it "pushing" the stalk away from you for full time mainbeam, or "pulling" it in to flash someome)
Same with the reverse lights.... use the 3 way switch, middle contact goes to the relay, then each of the other 2 outer contacts, one goes to the reverse switch on the gearbox (make sure it's the "lamp" side of the switch, not the positive side from the fusebox, otherwise the reverse spots will always be on!)
The other contact on the switch could go to a permanent feed from the fusebox/dash area, so you can flick the back lights on for working/revocery etc, or put the switch in the middle "off" position to turn them off completely.