I believe you're looking for an allen head ratchet set, like the one below. A trip to your local hardware or home improvement store might be in order so you can see some products to get a sense of whether or not they can fit in the small space you have before you make a purchase.
(http://chapmanmfg.com/ , also available on Amazon)
I've never used this particular manufacturer's tools (and this kit might be a little expensive for a one-time job) so I'm not necessarily recommending this particular kit, but the kit includes a low profile ratchet that may work well in a tight space. There are other kits out there that use a similar small ratchet.
Many ratchet sets use a standard 1/4" ratchet with an adapter socket that takes the allen wrench insert, so it ends up being a little bulkier than this ratchet above appears to be. Here's an extreme example, this looks like a 3/8" ratchet with a 1/4" adapter on it, then the 1/4" adapter has a hex bit adapter, then the hex bit is on the end:
Another option - flexible drive for a socket wrench:
If you do get a standard ratchet with a 3mm hex bit, then you might be able to use a flexible drive for your ratchet to get into a tight space -- it can bend around obstacles while still being able to turn a fastener:
(Image from Amazon)
One more option - a ball-head allen wrench:
Another tool that might be useful would be a ball-end allen wrench:
(image from Amazon)
The ball-end lets you insert it into the bolt at angle instead of having to go straight in. However, the ball-end doesn't fit in the fastener head as securely as a standard squared off wrench, so you may not be able to give it enough torque to break loose a tight fastener. Here's a closeup of the ball-end: