Design or analyse your favourite coaxial pipe.
Use of substrates is a common practice. Here you can design and analyse your substrate lines.
Edge coupler are common for obtaining alow power sample of a high power line.
Using stepped microstrip or coaxial lines allow broadband resistance transformation.
Split or combine in phase power. Choose the best sum point for your desigh instead of the default.
Combine and transform three power amplifiers into a load.
Combine and transform three power amplifiers into a load with intrinsic low pass filtering.
At some stage capacitors with hign power capability willbe required.
Likewise, at some stage your high power filter or matching element wi8ll require a power inductor.
Forget sensitive in phase combiners that blow when an amplifier fails. Use a quadrature hybrid instead, better matching, better performance on amplifiers module failure.