Ok, there are 2 ways to do targetting when multiboxing:

1) Have a key on all the alt toons that says "/assist toon1" where toon1 is your main. This way, you run around and click on a mob with you main, tap this key, then hit '2' or whatever your DPS spell is on all the characters. The problem with this is that if the main ever dies, your whole group is toast.

2) The whole "focus" targetting system. What many people do is they have F1-F5 keys dedicated to macros that say:
/target toon1;
/focus

This is generally considered the best way to do things. I use this setup.

So when I log in, and have keyclone all set up, I tap F1 and all my toons now have their focus set as my main.

Then, all you have to do is macro all your DPS spells like this:

/cast [target=focustarget] Fireball

This makes it so that whatever your main is targetting, the alts will cast straight to that without having to /assist first.

The beauty of this strategy is that if your main dies, just hit F2 and click over to another screen to begin controlling the 2nd character as your "main".

As far as healing goes, what you want to do on the priest is use the numberpad on the right of the keyboard. I'd simply do this:
- Set the 1 key to "/target toon1" which is yourself
- Set the 2 key to "/target toon2" which is the 1st lock
- Set the 3 key to "/target toon3" which is the 2nd lock
- Set the 4 key to Renew (no macro needed)
- Set the 5 key to Flash Heal (no macro needed)
- Set the 6 key to Greater Heal (no macro needed)

This method, makes it so that when you want to heal, all you do is tap whatever target you need (1,2,or 3) then cast the appropriate heal spell (4,5, or 6).