"Leona, Thresh and Morgana are still dominating bot lane?

Your title is misleading. Not all leagues have the same top 3. The 3 you described are actually lower elo like bronze/silver. Higher elos like diamond have janna leading the charge nami and thresh.

pretty sure its janna and nami dominating bot right now

Sona has a pretty high win rate though.

Although definitely, those three need nerfs Leona - I'd settle for ult cast range reduction Thresh - do less damage, he already gives so much utility. Morgana - rework or change Q and E. Make DoTs no longer proc Spellthief per damage tick, just once per spell cast per champion that takes damage.

I think Leona is fine. She is all or nothing and relies on her team to follow up. She has no poke and her cc is melee except for her ult. The only thing I could see them doing is either hitting w resists or ult cd if they felt she was too strong. Unlike most supports shes melee and it feels terrible to be a tank behind.

Thresh as been a problem always. He started overloaded and still after nerfs he has utility that can be shaved off. His damage is decent for most of the game because because of his passive and the charge up damage of his auto which is perfect for poking in lane.

Morg just needs a more accurate hit box on q and maybe a lower snare duration for earlier levels.

Janna seems to be close to a good spot. She works well disengaging. Maybe shave off some of the healing from her ult or sheild cd to be raised.