How to calculate moments with 3 separate weights of different amounts at different points?
200n 100n 300n

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what would the moments be?

I don't completely understand your drawing, although I can see that you certainly
did put a lot of effort into making it.  But calculating the moment is easy, and we
can get along without the drawing.

Each separate weight has a 'moment'.
The moment of each weight is: 

             (the weight of it) x (its distance from the pivot/fulcrum) .

That's all there is to a 'moment'.

The lever (or the see-saw) is balanced when (the sum of all the moments
on one side) is equal to (the sum of the moments on the other side).

That's why when you're on the see-saw with a little kid, the little kid has to sit
farther away from the pivot than you do.  The kid has less weight than you do,
so he needs more distance in order for his moment to be equal to yours.