Why can't pgAdmin3 connect to Postgres?
On Unix, when psql is used without the -h option or a PGHOST variable set, it uses Unix local domain sockets, which corresponds to local in the TYPE column in pg_hba.conf and no ADDR column at all.

That differs from putting localhost in the host field of pgAdmin's connection dialog. This one corresponds to host in the TYPE column and 127.0.0.1 in the ADDR column or whatever localhost is resolved to by the TCP stack (with IPv6 it could be ::1 for instance).

In general, this is what is confusing when psql and pgAdmin connection attempts do not trigger the same rules in pg_hba.conf. If that's the case in this question, try to leave the host field in pgAdmin empty.

On top of that, Windows does not have Unix local domain sockets so psql on Windows defaults on localhost as if -h localhost was used.