My Gerbil is biting me HELP!!!?

I got 3 female gerbils 2 weeks ago and they wouldn't bit at all,but now one of them named Doodles has started to bite me every time i put my hand in their cage(critter trail.... are those good for them?) and the other ones don't do anything they even eat from my hand but not Doodles it doesn't even try to smell my hand it just bits my finger and wont let go. He use to be the most friendly but now is being the most aggressive. Please someone help me.