My regular game group uses roll-under 3d6 (or 4d6 or 5d6) stat checks for a variety of actions. As a player, I find it fun to roll a handful of dice on occasion. I also noticed that Holmes used 3d6 stat checks in the introductory RPG found he presents in his 1981 RPG. He calls them as "feats of strength", etc and uses them in lieu of saving throws.
Last year I asked on OD&D Discussion about the origin of this type of check. B/X instead presented a roll-under d20 stat check as a standard option. One poster told me the 3d6 roll was used in The Fantasy Trip. Holmes' book has a section on this RPG, which I hadn't looked at in a long time, which describes this exact type of check in Melee, the predecessor of TFT from 1977: "If a die roll is made for a hit (and this means rolling three dice and getting less than your dexterity score)" (pg 114). Furthermore Holmes writes: "I have used the "roll less than your dexterity (strength)" system in the practice game earlier in this book". Tracing influences doesn't get much easier than an explicit statement like that!
But what about roll-under 3d6 stat checks in D&D itself? In the OD&D thread, another poster pointed to the module "The Hall of Beoll-Dur" in Dragon #41, Sept 1980:
there is an original procedure for saving throws which is used in some
locations... roll 3, 4 or 5d6 (the number of dice varies) and subtract
one point from the dice roll for every two levels of experience the
character has attained. Compare the resulting number to a specified
ability (this also varies), and if the adjusted dice roll is less than
the character's score for the ability in question, the saving throw is
The subtracting levels of experience from the roll is an interesting variation and makes the rolls improve by level like ordinary saving throws. However, most groups that use stat checks seem to use them in addition to saving throws and do not improve them by level.
The Dragon #41 reference was the earliest for D&D that I knew of until yesterday when Wayne pointed out that the module B1 In Search of the Unknown by Mike Carr has at least one 3d6 check. I looked at the original
version, for Holmes Basic, and found one in the infamous Room of
Pools. It's a 3d6 Con check vs the effects of wine, with a failed
roll indicating intoxication for a number of hours equal to the
difference between the result and the Con score. This is just one of a mish-mash of ability score influenced actions in B1, like much of D&D at the time, so the general utility of a 3d6 isn't at all apparent there. But it's interesting to see it appear in an actual Holmes Basic product.
This is now the earliest 3d6 check I've found in a D&D product, thought I wouldn't be surprised
if one shows up earlier, such as in the zine Alarums &
* * *
Update: I remembered reading on Dragonsfoot about a few 3d6 stat checks in the Player's Handbook. This book came out in 1978 several months before B1. As with the B1, they are very situation specific:
"Any creature at the edge (1') of such a pit uses its dexterity score as a saving throw to avoid falling into the hole, with a score equal to or less than dexterity meaning that a fall was avoided" (pg 76).
Phantasmal Killer spell:
"Note that the saving throw against this spell is not standard. The
subject must roll three six-sided dice (3d6) and score a sum equal to or
less than its intelligence ability score in order to disbelieve the
apparition. The dice score is modified as follows..."
Update #2: Another one I remembered reading about on Cyclopeatron. The Underworld Oracle #1 (UK zine) from 1977 describes a 3d6 Int check versus an Apparition:
"Therefore, even if the victim is aware that this vision cannot physically damage him, the suggestion is so strong that he is forced to roll 3 x 6 die. A result which totals under the victim's intelligence, means that he is free of the creature's suggestion..."
Update #3: I just found the original description of Phantasmal Killer in the OD&D Illusionist class write-up by Peter Aronson in Strategic Review #4, Winter 1975 (transcribed here), and it also contains the 3d6 roll-under Int check. So this now the earliest one I've found, beating even Melee.
"The subject may try to disbelieve it, to
do so he must roll his intelligence or less on three 6-sided dice" (followed by a number of situational modifiers)