Calculating capacitance for three conductor transmission line
Assuming the insulating material surrounding the smaller conductor is of uniform thickness, the capacitance per unit length between the large copper conductor an either of the small copper conductors is:
\$C_{large-small} = \frac{\displaystyle 2\pi\epsilon}{\displaystyle \ln(b/a)}\$
where
\$a\$ is the outside diameter of the smaller conductor
\$b\$ is the inside diameter of the cavity in which the smaller conductor and insulator reside
\$\epsilon\$ is the permittivity of the insulating material between the smaller and larger conductor.
The capacitance per unit length between the two smaller conductors is found by calculating the capacitance of two of these capacitances in series, which is one half of the figure above.
\$C_{small-small} = \frac{1}{2}C_{large-small}\$
or
\$C_{small-small} = \frac{\displaystyle \pi\epsilon}{\displaystyle \ln(b/a)}\$