Let's say every character (3 characters) at my location must make a Fortitude 12 check.
As Lem, can I recharge three cards to give 1D4 + 3 to each check?
Could I throw three blessings as well? (1 to each character)
I'm just a little confused about simultaneous checks and power / card usage.
Yes, because there are no simultaneous check. You resolve each one, one at a time. If an order isn't specified you choose the order.
Remember that each player may not play more than 1 card of each type or use any 1 power more than once during each check.
Finish One Thing Before You Start Something Else. You do many things in a specific order, and you need to finish doing each thing before you do the next thing. You move before you explore, not after. If a spell used in a check can be recharged, finish the first check before you attempt your check to recharge it. If a villain requires two sequential combat checks, finish the first before starting the second. Don’t start a new process until you’ve finished the last one. (That said, if the game doesn’t specify an order for things, you decide the order.)