First of all there are 5 errors in validation. You should fix those so they don't add to the problem. The very precise width and margin definition are probably messing up the floats. If it is too tight it will break. You need to keep the totals with slightly less then the available space in the container.
The three latest news blocks are all inside the same parent which only has a 33% width, and they are not floated and do not have a width.
The parent must be wide enough to hold them. They each must have a width specified. The total width of the three must not be more than the width of the parent block. Each of them must have float:left included in their styles. The parent does not need float:left
I am trying to use the 1140px grid system so I have tried to apply twelvecol to #latest-posts and fourcol to the #latest-post. Problem is im using code someone else gave me so I guess im lacking the understanding of how it was put together