The 4th Aliyah of this week's parshah, Mas'ei, lists the 12 tribal heads who are to be in charge of dividing the land during the time of Yehoshua bin-Nun.
The first 3 tribes listed are Yehudah, Shimon, and Binyamin.
From the 4th onward, all of them have the word "Nasi" in front of their name.
The grouping is obvious: Those 3 tribes would, after the time of Shlomo, be the Kingdom of Yehudah, and those 9 would be the Kingdom of Yisrael.
I didn't see any peirushim on this in Mikraot Gedolot.
Does anyone have any drashot on this?
And while we're at it, does anyone have a drash as to why they are called the 10 Tribes (which is a title given by Hashem in Melachim 1 11:35) when it is only 9?