Triplexer matching circuit

There is no magic "frequency selective matching network" available. You have to design the three filters to compliment each other. Investigate "singly terminated" filter design for a contiguous triplexer.

Some advanced filter design programs have a triplexer function.

In general, as stated in the above post, you design a singly terminated filter for each one but of the type that has the input impedance go towards infinite out of band. That way you can parallel the inputs without their ruining each other's response or the total input impedance.