After praying at all three Totems and unlocking every single Werewolf perk, when I press Z button I only activate howl that makes my enemies flee. How do I activate two or three other howls?

The last totem you prayed to is the only one that you can use in beast form. For some reason, Bethesda didn't allow the werewolf to access the favorites menu to change unique abilities like the vampire can.

Since you're on PC, though, there's [url=http://skyrim.nexusmods.com/mods/12687]a mod[/utl] that can help (requires SKSE for the ability to change howls while in beast form).