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In a stage three arithmagon the number in the top vertex can be "a". The bottom right vertex is "b" and the other vertex is "c".
First: Add all the numbers written in the rectangles and divide the sum by two.
For example: If the numbers in the rectangles are 20, 23 and 35, then because the sum of any two vertices equals the number written in the rectangle between them
a+b=23
b+c=20
c+a=35
_________
2a +2b+2c=78

Therefore a+b+c=78/2=39

Then: solve for each vertex.
c= (a+b+c)-(a+b)=39-23=16
a=(a+b+c)-(b+c)=39-20=19
b=(a+b+c)-(c+a)=39-35=4