Unlikely there is a third book, but even more unlikely that there would have been drafts around. Lee was traumatized, not in a “I can’t go on living” way but still traumatized, by her sudden and near universal praise for her book. She stayed busy in the literary world, but could not bring herself to write again.
So even if she did write, it likely would have been a single draft that she kept out of view from anyone else. Like Emily Dickenson with her poems. Only after her passing was the volume and quality of her work revealed.
Some people cannot handle the demands and expectations of fame, not that I’ll ever know about it firsthand.
The famous have various ways of tolerating their status. I recall reading that twenty something years ago, Diana Ross had a clause in her contract that anyone not actively working with her was not to even look at her. I’ve heard the same things about Hillary during her time as First Lady.