converting design into 3 section coupler

Hello, i have implemented the design of the coupler presented in the attached article ,but it works only till 10Ghz, i need to change it into a 3 section version in order to make it work till 20GHz.are there any formulas of the geometry of switching it into 3 section coupler? (the article only goes to specifics regarding a single section couple )
Thanks

01390982.pdf