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?
 A: In your question you say you want a circle that connects the points A and C.  If you are not including the point B then the circle can be any diameter all the way from infinity to a diameter equal to the distance from A to C.  
If you want the circle that connects all three points then find the midpoint of AB and then the line that is perpendicular to AB and intersects the midpoint of AB.  Then find the line that is perpendicular to BC that intersects the midpoint of BC.  Where those two lines intersect is where the center of the circle that joins AB and C.
A: 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. 
