I only transfer clients into HB from the prior year when I start working on their return, keeps all the dead wood out of my current years Homebase and my HB is easier to manage.
Or prior to transferring any clients into 2020, go into PS 2019 and organize your HB by 'Last modified date' (just click on the HB column heading) then you'll see all the people that you didnt work on all at one end of the list. Delete all those dead end clients and then transfer whomever is left from 2019 into the 2020 tax year.
This wont help with the 3 year part, I dont know how you would take the 3 years into the scenario.
Try looking at the status for those only transferred. You can go into the return and look at the 5 year history.... as an alternative you can create a version of home base that shows a column for your total fees per client for each of last 3 years... if you didn’t invoice them, you probably (hopefully) didn’t prepare the return.