Q:

How kafka partitions behave

Can you explain how kafka partitions works for this scenario
If i produce 9 (1-9) messages round robin with 1 topic & 3 partitions.
Does it means that:
Partition 1 contains: [1,4,7]
Partition 2 contains: [2,5,8]
Partition 3 contains: [3,6,9]
?
Also how many consumers can get all the data 3? why?
Can you explain?
I guess also that consumer group can solve it but not sure why

A:

Can you explain how kafka partitions works for this scenario

Your understanding is correct.

Also how many consumers can get all the data 3? why?

Depends on how many consumers you have in your consumer group. 
If you only have 1 consumer in a group, it will get all the messages from all partitions. 
If you have 2 consumers in a group, each will claim a subset of the partitions, e.g. 1st consumer will get all messages from partitions 1 and 2 and the 2nd consumer will get messages from partition 3.
If you have 3 consumers in a group, each will get one partition assigned.
If you have more than 3 consumers in a group, 3 consumers will get one partition each and the remaining consumers will not get any messages, just act as redundancy in case of failover.

