I am new to chess and am confused on what to do with the following opening

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    1. e4 e5
    2. Nf6 Nf6
    3. Nc3 Bb4

I have been playing a3, but the Bishop then proceeds to take c3 so i end up with a doubled pawn.

If someone could offer some guidance or tell me what this opening is called so i can do some research, it would be very much appreciated.

Cheers