Circle diameter using only 3 points I've never been very good at math and confuse very easily so I need some help in doing a calculation regarding a circle.  
Let's assume we're drawing the letter L.  
The top point $A$ of the long leg is at $0$ the bottom point of the long leg $B$ is at $3$ feet and the point to the right $C$ is at $6$ inches.  
What would the diameter of a circle be if it was drawn so that $A$ and $C$ were connected?  Can you tell me in lay terms how to make that calculation or some semblance of usable formula that I might comprehend, please?

In your special case, you have to use the Pythagorean theorem, as the letter $L$ has a right angle. 
You have then $AC^2=AB^2+BC^2$
And $AC$ is the diameter of the circumcircle.
For a more generic solution (working with any three points), you will need to find out the coordinates of the center $O$ of the circle, and the radius will be for instance $OA$. 
Finding the center of the circle relies on the median of two of your segments $AC, AB$ and $BC$, whaterrve your choice. 
