http://wiki.spiralknights.com/Energy_FAQ

The energy sharing scenario should explain everything but I'll explain your specific situation.

You have 3 accounts, 1,2,3
Each account has 3 knight slots,
1a, 1b, 1c,
2a, 2b, 2c,
3a, 3b, 3c

knight slots under one account always share mist even if you pay and are on seperate computers. If you spend some with knight 1a, the knights 1b and 1c will also see a loss in mist.

With unpaid accounts on the same computer mist is also shared. if someone uses any knight on account 1 and later someone else uses a knight from account 2 on the same computer, all 3 accounts see a loss in energy. That includes account 3 who didn't play that day. If you pay, an account will get its own unique mist pool. All unpaid accounts then keep sharing the public/computer mist pool.

Since two accounts are paid, they both have a unique mist pool, leaving the unpaid one with the public mist pool. None of YOUR accounts share mist,
But if a family member or friend uses your computer to make a fourth free account that's not yours, it will share mist with your unpaid account.