Two Column Loop with One Big Post on Top?

Hello!
I am trying to create a template where I have say a 600px wide div
and the first post will be 600px wide,

and then beneath it, the next two posts would be 300px wide and floating next to one another so it would look like this:

_______________
| POST 1 |
_______ ______
|Post2| |POST3|

How should I code the loop if I am trying to achieve this?
Thank you so much in advance.

Cheers!