The most popular way to play with multiple humans controlling one player is to make one person control the army and let another control the economy.
Voice chat is especially helpful to coordinate.
Now, if you want to put 3 or even 4 humans on controlling one player, you will run out of tasks to divide.
It does not work to give three players two knights each to control.
I would recommend a double or triple TC start where each player controls one TC’s economy or army.
Upgrades are shared across the one player, e.g., only one TC needs to advance to the next age.
This way, you have more tasks to go around.
There are a number of maps with multiple TC starts, but the most popular 2 TC start is the Budapest map and the most popular 3 TC start is Metropolis.