HI soccermomlynnette,
I (and I think every person on this forum) struggle with this same dilemma, of wanting to take on more contacts than I can afford to support. I have realized that I am only one of many people who are supporting our troops, so I can let a few contacts go by and hope and pray that someone else is helping them out.
The other thing I am doing is writing a LOT more letters, and sending less large packages. I really do love to write letters and send cards, and for someone who isn't getting any support, I figure a letter telling them "thank you" and "I care about you" is worth more than warm socks and beef jerky. (Not that I will stop sending socks and jerky, of course!) So if you do see a contact that you want to reach out to, but can't afford to send a package, I highly recommend writing them a letter. And I'm assuming by your screen name that you have kids, so a letter and/or artwork from your kids is even better!
Otherwise, you are absolutely right in sticking to your 3 contacts and supporting them.
This is my take on it, and it is an evolving process for me. So do what works for you and your family. 
Actions speak louder than words.
Pictured in avatar: Katie, Ashlea & Emily (three of my nieces)
holding a flag that was given to me by one of my soldiers.