I have (3) five inch square posts that I want to cover with cedar shakes.
How do I measure and cut the shakes so they fit on the post? My geometry skills are not very good for triangular shaped shingles.

 

You will install the shingles on the two opposite sides (north and south) first, ensuring that you are keeping one side square with the other, not raising one side higher or lower. I would first mark the top edge of each course with a framing square, making marks on all 4 sides of the post to layout exactly where every row will go. A homemade story pole might be helpful since you have so many posts to do. The shingles on the initial sides you install will be square on the ends, and flush with the post (or slightly longer). Once those sides are installed, you should be able to easily figure out the size needed on the other sides (east and west) by measuring the width between the bottom side of each shingle (the long dimension) and the width across the top side of each shingle (the short dimension). The difference in the 2 measurements divided by 2 will be the amount of taper you will cut off each side.

The other way to do it is to put up one row of shingles on the north and south sides, then install one row of shingles on the east and west sides leaving them a bit long. Take a router with a piloted flush trimming bit and router off the excess shingle on each side. This will achieve a perfect fit each time. Then proceed to the next row of shingles on the north and south sides and repeat.