"Before I come" or "Before I came"?

You would say "before I came" in that situation.

If you were to say something about going to country B in the future, you would say "come"

for example:
"After I come to country B, I am going to cancel my mobile contract."

Came Come

before i came is saying your there already but your thinking back to when you werent, before you come is your still in country A and planning on going to country B

Before I came, the other one is talking about the future.