Hi,
I am a 15 year old girl and started my period 3 months a go. I have only received 1 period and missed the other two. Is this normal?

This is very common when a girl first starts her period. It can take several years before they become regulated.

Very much so. Your body is still trying to get into the cycle and so things may not work like clockwork or may be on and off for months or even years on end.
But always have your supplies on hand for any emergency spillages!