After the takeover of Qin at Sanyou, the preparations for the Coalition Army against Qin is already set. The WAR of Qin against the six remaining waring states is where the whole story or the whole season 3 of Kingdom would revolve. The season 3 of Kingdom anime is the main event of the manga series, I think.
I am quite sure that if ever Kingdom would have a season 3, its fans circle and recommendations would drastically increase right before the end of the coalition war. The season 4 of the story is quite hard to tell because it would start with the death of the King’s younger brother of Qin and then a war inside the Qin’s faction led by the Qin’s Chancellor to take the throne out of the King’s reach.
Maybe I am jumping into conclusions now since I already read the manga. But yeah, if you are a Kingdom anime fan then it is nice for you to read the original manga. I am pretty sure that the creator of the manga would be glad to know that the anime fans are moving onto reading his own manga.
A season 3 for Kingdom anime? Yep but the fans has to wait a little bit more since it takes time to create a long running anime with 30-ish episodes per season.