Hi @Guillaume_Khayat, thanks for the question! The connection pool is on a per-mongod/mongos basis, so when connecting to a 3-member replica there will be three connection pools (one per mongod), each with a maxPoolSize of 1. Additionally, there is a required monitoring connection for each node as well, so you end up with (maxPoolSize+1)*number_of_nodes TCP connections, or (1+1)*3=6 total TCP connections in the case of a 3-member replica set.
Also just to note, maxPoolSize is the correct option to use with the unified topology, you should be able to safely drop poolSize which is only used by the legacy topology.
Please let me know if that answers your questions or if you need any further help.