Omega theme responsive grid two break point
Omega 3 uses the 960 grid, but you can alter it adding a custom 960.

Go to http://grids.herokuapp.com/ and design your grid and override the default one (on a sub-theme).

But I strongly recommend going with Omega 4. Is far more powerful and it is designed to give you the freedom to use any grid or presentation framework you like! If you don't like sass (bad for you) you can just alter the css directly.

Edit: As @Clive mention it, the Omega 5 alpha is out there! It's fantastic and follows all the html/css best practices + complies with D8 standard. Go test it, it's almost stable and your themes will be ready for D8.

Disclaimer: It has been ages since I use the Omega 3.