I just finished watching Hotaru no Hikari seasons 1,2 and the movie, does anyone know if there are plans or rumours of a sequel?  I believe there is the Hotaru no Hikari BABY  manga that is yet to be adapted. I really hope we get a third season in 2020.

You're right, the Hotaru no Hikaru Baby manga is currently being published by Kondansha and there are 3 volumes out.
Because I also love this drama, your question inspired me to do a lot of googling. Unfortunately looks like there's no news or plans for a possible 3rd season.
But you never know, maybe someday!