what the initial 3 daemon of postmaster process do?

Hi All,

After fist installation, using pg_ctl script, I started postmaster process.

after 'ps -ef|grep postmaster', there are 3 daemon process in its initial state.

I like to get information about the reason belows.
1. why 3 daemon processes start in the starting point
2. what the role of each 3 daemon processes is
3. and how can I control the initial daemon process count.

I like to also confirm whether the count of postmaster process that is up in the run time 
increases or decreases according to the client-request, that is, the DB transaction.


regards, Dohyung Kim