oh, are your rats Espresso, Latte, and Mocha?

Have you been feeding them foods that they can snatch from your fingers if they want to? I've noticed that when I get into the habit of giving mine some foods that they can take, they sometimes mistake my hand for food. When I squeal, they back-off and are usually careful to be gentle next time.

Yes, fingers smelling of food or rats 'trained' to expect food in hands near cages is often a common reason.

I made a firm rule to stop feeding mine through the bars a while back (about 5 years ago, now) when one had his nose in the corner of the cage sniffing away - looked so cute I couldn't resist going up and sniffing back - bang lil teeth go straight into the tip of my nose, I had to walk around with a bite cut on the end of my nose till it healed And you can't say that noses smell of food. Well, maybe they do. Maybe my breath did?

It sounds like you've only had them a week or so? They could also be trying it on for dominance with you.

As I put in my thread just lately about my being bitten yesterday, and as haleyboo says, squeak or eep when they bite rather than nibble, because you're speaking their language. It's very effective. Once I did it after being bitten quite badly, and the rat actually apologised by lying on his back under my hand and not moving - the equivalent of saying, "OK, you're in charge, you can see now I'm not going to contest you for leadership." I was gobsmacked and wondered what the rattie for "it's OK, don't worry about it" was!

Another reason might be their sight.

Are they pink- or ruby-eyed? If you have bright light, that can prevent them from seeing you coming. They have pretty rubbish eyes at the best of times. Dim the lights if so. Also, I realised that talking to them as you approach isn't much use - your voice and your hand are in two different places, so it can confuse them if they're only relying on sound. You can click your fingernails back & forth (thumbnail to fingernail) as you approach, and this gives them warning.

Be patient, hold your ground, but most of all be brave - it does take guts to overcome the fear of having those sharp little teeth aiming for your poor little fingers, but it can be done. Good luck, Tiffany, and report back!

If they're biting because they think they're getting a treat, you can train them to know when they're getting one (it's really easy, too). When you give them a treat, say the word "Treat!" (or make a noise you always will use). That way, they know if they don't hear the cue, they aren't getting a treat to snatch.

 
I trained my rats to come to a sound for treats without even realizing it. Really, I just can't stand to be quiet, so I found myself making the easiest sound everytime I wanted them to come to me (which was usually only when I was going to give them a treat). After a while, I realized that they were not just cautiously poking their little noses out of the hiding places when I made the sound, but were RUNNING. BTW, the sound was a really high-pitched kissing noise. They responded better to the high-pitched one than the low-pitched one (because of their hearing frequencies?). They really are very sweet and I was blessed with wonderful first rats.