I love the three sisters...hey, if it worked for the Native Americans, it works for me, too!
It might be too late for me to tell you, but I here is the easy way to plant that has always worked well for us.
First, plant your corn with a spacing of 5 feet between each stalk. Plant the rows about 8 feet apart. When the corn is about 6 inches tall, plant the beans right beside the corn. You can do one or two bean plants, depending on what sort of yeild you need. At the same time that you plant the bean seeds, plant the pumpkin/squash (whichever) in a row of its own in between the two corn rows. Space each hill 5 feet apart also.
If you don't like row gardening, you can plant the corn and beans together using the same time schedule and just space them 5 feet apart in all directions. What you would consider to be the "center" plant in the circle is where you plant the squash.
This worked well for us everytime we did it, but we now have gone to square foot gardening, which requires no weeding and no wasted space. We still do a variation of three sisters planting, though. Good Luck!