The three O-H bonds involved in hydrogen-bonding form a perfect triangle. The other O-H bonds all are pointed "up". There also exists an isomer of this trimer in which two of these O-H bonds are "up" while one of them is "down". Can you imagine that structure? There also exist non-cyclic or "open" structures of water trimer but they are slightly less stable. Finally, note that three alcohols also could form such a trimer.