Does drawing the tentacle chaos token imply a failure with test result 0?
Yes, the tentacles token is an "automatic failure" for the skill test, which is defined on page 5 of the RRG:


  If a skill test automatically fails, the investigator’s total skill
  value for that test is considered 0.


So in your example with a Willpower(3) test, revealing the tentacles would deal 3 horror.