Question

Can you get the professor's other two pokemon throughout the game?

Answer 1

Yes, you can, if you friends got HG or SS and do eggs with DITTO and they trade them, you can.

Answer 2

You don't say which game you're playing, but no, you can't get the other two starters during the game (in most games your rival gets one of them anyway).

However, in some games like HeartGold/SoulSilver, you can choose one from a different set of starters, e.g. Bulbasaur/Squirtle/Charmander. But you still only get one of the three, not more.

You need to trade with others to get the rest.

Comments

The reason I didn't specify is because I was asking for all the  Pokemon games.

Answer 3

Only ways to get other starters are hacking and trading.