1. This just means that A is somewhere east of B. Mongolia is east of Portugal. Really far east, but east.
3. I would think that A is not very far east of B. Like: Do you see B on the map? A is to the east.
2. This sounds the least natural - which is why I put it last. Either A borders B to the east, or A is within B, on the eastern side. But this could be more clear: A is