is it normal that my girlfriend loves me already

Oh my... I think she's rushing things and if I were you I'd be worried. What if she turns obsessive? I think three weeks is too soon to say,"I love you", though, sometimes people can just feel that the person they are with is "The One". It would be a whole other story if the two of you had known each other for a long time or loved each other from afar, etc. I think you should calmly take her aside or tell her in a public place (not your school) that you think she's taking things a tad to fast and that its making you very uncomfortable.