It's impossible
See this site for an explanation.
If you just want it reasonably close, of course, just measure the circumference and divide by three, but I suspect you have something else in mind.
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The whole thread (37 posts)
- Question: how do you find 3 points on a plywood circle 120 degrees apart, without a protractor? (n/t)
- Without a template, you put your anchor bolts in. Drill out the plate.
- Yet another method
- Here you go, have a protractor:
- If I remember enough geometry...
- Cut a circle out of paper, exactly the same size as your plywood circle
- Yes, there is a way!
- So... tell us what you're building! (n/t)
- Why can't you use a protractor?
- It's impossible