Connect to PgAdmin III via localhost
The basic misunderstanding: "localhost" is not a local connection. Your "trust" line in pg_hba.conf is not applicable for your connection:

local   all   all   trust


That only applies for connections via Unix-domain socket. Best choice for you is to connect via this route. Per pgAdmin documentation:


  The host is the IP address of the machine to contact, or the fully
  qualified domain name. On Unix based systems, the address field may be
  left blank to use the default PostgreSQL Unix Domain Socket on the
  local machine, or be set to an alternate path containing a PostgreSQL
  socket. If a path is entered, it must begin with a “/”. The port
  number may also be specified.


Bold emphasis mine.

More about connecting without password in this related answer on SO:
Run batch file with psql command without password