Minimum 3

Minimum 3 is an informal reference to the GURPS rules for success checks.
==basics==
B8 basically requires rolling under the skill. Since the minimum roll is 3 and you must roll equal or less to succeed, obviously one must have a skill of at least 3 to have any chance of success. Someone can't roll 2 or less on a 3d6.
B343 however mentions "Regardless of the score you are rolling against, a roll of 3 or 4 is always a success". This would apply even if you had skill 2 and rolled a 3, or had skill 3 and rolled a 4, in either case not being equal or less (both are over).
==when to roll==
===Exemption for defense rolls===
B345 however, then limits when rolls can be attempted at all (ie meaning you can't succeed with a 3 or 4 if you can't roll to get them to begin with):
:You may not attempt a success roll if your effective skill is less than 3, unless you are attempting a defense roll (p. 374).
374 then includes an example of that:
:An active defense roll of 3 or 4 is always successful – even if your effective defense score was only 1 or 2!
It's unclear if there are any further examples of rolling against a defense score of 0 or a negativen umber.
This means (except for active defenses, for unexplained reasons) at least 3 is needed for other things. For example:
*attacks
*feints
*perception
Other things exist in a vague area in Basic Set. For example:
*resistance rolls
*resisting in a quick contest
===Exemption for resistance===
The errata finally clarified that resitance are included in the "defense roll" category despite not being an "active defense":
http://www.sjgames.com/gurps/faq/FAQ4-3.html#SS3.1.3
3.1.3 When I roll against a target number less than 3, when do a result of 3 or 4 still succeed?
:Defense rolls may always be attempted, and a 3 or 4 is still a Critical Success.
:Same thing for Resistance rolls to magic or poison, most forced IQ and HT rolls, and any other resistance roll against a force directly targeting a living or sapient being.
:Other rolls (such as skill rolls or physical feats) are impossible if the target number is below 3.
:However, the dice may still be rolled if the character does not realize the attempt is futile,
:and if you're rolling vs. a -7 or worse, even a roll of 3 will be a Critical Failure.
Category:Rules