Particularly with your setup (3boxing) and the recent changes to XP gained from questing, you should DEFINITELY quest as much as you possibly can. Collection quests shouldn't be too hard for a tri-boxer either, but you can pick and choose depending on the drop rate.

I had a great time with locks just running and dotting things on my way to different quest objectives. Stay off the road and just make a macro that sends in your voidy while you dot things up. Just run straight past them and let the dots tick away. It gives you that little bit of extra XP.

Don't forget to keep your soulstones up on your priest.