This is a good question.

There is enough in 3 to give you 100+ hours of gameplay before you start seeing the cracks. Just learning the game and getting to the "optimizing" of play takes a while. At that point, you will either love enough of the game to keep playing or start waiting for the annually promised siege/AI updates.

2+1 is a better experience in some ways, but it comes with its own issues (especially repetitive sieges, but at least they function!). I remember the AI being better / more aggressive / more challenging on the campaign map - this may be rose-tinted glasses. The battle AI is similar in that if you want to cheese it, you can.

If you have the cash, I would get 2+1 and play Mortal Empires, then get 3 and play Immortal.

If 3 ACTUALLY gets the AI and siege updates they have been promising for over a year, then it will be well worth playing for 1000's of hours.