Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. g3

This opening line is uncommon among master games (it has only been played around a hundred times) so it doesn't have any theory attached. The extremely basic idea is that it opens up the g2 square for the bishop, which attacks the knight on d5 if it is left undefended. This can be used to regain the tempo that is commonly lost by playing g2 in other openings.