Depending on how much space you have and how bright the sun is, you can choose from several ways of laying out your three sisters garden.
Step by Step:
In options #1 or #2 above, you start by making mounds for the corn clusters. Each mound should be 18 inches across and about 5 inches high (if you can bury some compost or kitchen waste under the mound, so much the better.) The mounds should be 5 or 6 feet from center to center. ( This is not terribly exact, so make it work with your space. The point is that they need about as much space as you take up with both arms spread.) Make the center lower than the edges.
In #1, the circular garden, the first set of mounds make a cross (in the four directions traditionally) with each mound 5 feet apart. The next set goes in the center of the wedges formed by the cross.
In option #2, just make the same 18” mounds down the center of your bed and 5 feet apart.
Now sow the corn: In the mounds, make four holes in a square pattern 6 inches apart. Put a corn seed in each hole, 1 inch deep.
In rows (option #3) plant your seeds 10-12” apart in the rows, and 1 inch deep.
Sow the beans when the corn is about 8” tall. Before sowing, hoe up some dirt to cover the bottom of the corn plants an inch deep. This will help them be steadier. Plant 4 bean seeds in between the corn plants, on the sides of the square in your mounds.
Layout #3, plant a bean seed 3” from every other corn plant around the edges.
When the beans have sprouted, its time for squash. Make mounds just like the ones you made for the corn/beans. Plant two squash seeds in each mound. You may need to hoe the entire area before doing this, because by now there may be weeds. Once the squash gets going, it will prevent weeds from coming back. Layout #1 and #2, place the mounds in between the corn clusters. Leave one side free of mounds for access.
Layout #3, the squash is in rows apart from the corn, to the south, east, or west, not on the north side. Plant squash seeds 3 feet apart in the row, with 3 feet between rows.