mean distance between three point

I have a question that I would like your help solving.

Let say I have three different points $a,b,c$ and a radius $r$.

Let say the $A,B,C$ are the circles with the centers $a,b,c$ respectively and with radius $r$.
Now, I know that the point $b$ lays in the circle $A$, and the point $c$ lies in the circle $B$ but not $A$ (meaning that $c$ is in the lune created by circles $A$ and $B$, $c$ is in $Bsetminus A$).
I want to calculate the mean distance between points $a$ and $c$.

Thanks in advance!