Right-Margin extends beyond boundary

[Greyfish]

Hey I have a layout that is 900px wide and in my layout I have 3 blocks that are 280px wide and horizontally-aligned, each block has a right margin of 30px. So when you add all that up the total comes to 930px (30px more then my layout width) so there's not enough room for the third block because its right-margin pushes it down to the next row. How can I get rid of the right-margin on the last block or make my layout ignore the margin. Please help!

[KDLA]

Apply a class to that block, and apply margin-right: 0; to that class.

[Greyfish]

Thankyou for your help KLDA, that was my original thought but I was wondering if there was a way to do out with having to add special classes.