@Lauren did you book before or after baby was born? Just not sure what to put on booking.
Apologies, read wrong. I thought you would be four months. Checked with my sister who travelled to Spain with hers at 3 months and other than a passport she said it was fine. If you want to use a car seat on the plane make sure it's an approved one and you have to buy a seat. She said that she also took feeding items in a separate hand luggage for hers. Hope this helps.
I flew to America at 3 months and didn't need to take anything extra with me. If you're unsure you could call the airline.