if a monster \ animal companion has a main weapon (bite) and 2 secondary claws.
can it choose to attack but once, and consider it as a main (1.5 str) weapon >?
No. It may seem to make sense, but the option is not in the rules.
Quote:
If you possess only one natural attack (such as a bite—two claw attacks do not qualify), you add 1–1/2 times your Strength bonus on damage rolls made with that attack.
