There shouldn't be restrictions on how big your BB has to be in order to get on plane. But seriously I dont' think it's a good idea for a 3 mths old bb to be on plane.
Babies under 6 mths are weak and will get infected easily. Your baby will be trapped in a plane for couple of hours as the passenger might have sickness. It could be spread easily and I'm pretty sure your BB will get infected. But if this trip is a must, then I would preferr you bring your own water for your BB. The water on planes are horrible, absolutely not suitable for babies.