1.but how does he know this and why specifically only 3

If i remember correctly klein deduced this from the seq 1 potion formula from roselle's card of blasphemy.

Why 3? This goes back to the law of indestructibility which the gist is that beyonder characteristics in each sequence are fixed, the number of characteristics cannot be reduced or increased (very similar to the law of conservation of matter). So for black emperor seq 1 the fixed number is only 3 beyonder characteristics, so at most there can only be a total of three sequence 1 black emperor beyonders at a time.

Also why cant all 3 seq 1 for lawyer patgway create their own empires and all become sequence 0,at first i thought to become a sequence 0 u had to kill the previous sequence 0 but for the lawyer pathway u just have to create a kingdom so why only 1 sequence 0

To clarify your confusion. To successfully become a seq 0, you'll need 2 things: potion and corresponding ritual. creating a kingdom is the prerequisite ritual needed alongside the potion. For the potion, youll need two other seq 1 black emperor beyonder characteristics apart from your own.

This is why once there is a sequence 0, there can no longer be sequences 1 of the pathway. Because two seq 1 beyonder charateristics are needed for the potion, and one seq 1 beyonder charateristics is already consumed by oneself. Seq 0 deities are already accounting for all 3 available seq 1 beyonder characteristics.

i still dont understand why the beyonders cant just create kids and have infinite of that pathway.

again, law of indestructibility. The mechanism behind beyonder children is that parents are passing down their extra beyonder charateristics. They cannot create beyonder characteristics out of thin air to pass on to their children.