To be honest your threshold progression is pretty basic if you want to really work on that. Start with 10 min intervals (6x10 - do as much as you can). If you have done 4-5 move to 3x15, 4x15, 3x20. If you have done 3 your FTP is off or you need more basic work.
With threshold you need at least 10 minutes of interval time to see any substantial benefit from the workout. Less than that is merely maintenance work. You can also mix threshold with longer sst work - with sst 15 min intervals as a bare minimum, 3x20@90% should be quite a basic workout and try to push longer. With anything aerobic longer is better.
Edit–
The 3 week block for threshold is not enough. Try 6 weeks and 2 rest weeks. This maybe helpful for you: Threshold riding for beginners
My personal experience with a threshold as a complete newb to cycling and endurance sport.