I can only speak figuratively as I know very little PHP, but you would probably use variables.
In your loop, you would increase a variable for your width by one for each post. Then, with each post, check that variable and if it’s less than three (posts 1 and 2) they would use one loop coding, otherwise (the ‘else’ case) they would use a second code (for the third post and onward).
The implementation is similar to alternating colors for comments, I think. It seems easy if you know how to code PHP, but I know little of variables so I can’t give you code right away. I would if I could, though.
