Recently I have been thinking that it might be fun to try running some FLAILSNAILS games using my interpretation of 3 LBB OD&D as a base. Were any players interested in such adventuring, here is how I might run it.

1. Ability scores do not provide bonuses, other than +1 to missile attacks from dexterity scores of 13 or higher and +1 HP per hit die for constitution scores of 15 or higher.
2. Magic weapons do not add modifiers to attack or damage, though some monsters can only be damaged by enchanted weapons.
3. The best AC is 2. It should be obvious whether a given visitor counts as unarmored (AC 9), lightly armored (AC 7), moderately armored (AC 5), or heavily armored (AC 3). Shields grant another further bonus of one.
4. A per-session HP total using OD&D hit dice should be rolled. OD&D hit dice only use six-sided dice, with bonuses for partial steps (for example, a second level magic-user gets 1d6+1 HP).
5. Death & dying is handled as a saving throw at zero HP, with success indicating unconsciousness and failure indicating death (there are no negative HP).

Other than the HP totals, none of these considerations should require any real work. For example, an AD&D paladin in full plate with a magic sword and shield would be AC 2, deal 1d6 “magic” damage with the sword, and use the fighter’s attack matrix values.

Enemy numbers follow the same rules (limited range of ACs, hit dice using six-sided dice, etc). Even dragons have no better AC than 2.

I generally have PCs re-roll HP per session to represent recovery, so rolling hit dice is something everyone does at the beginning of the session.

Some magic items may function slightly differently. For example, in my interpretation, continual light enchantments cannot be placed on objects. These would need to be handled on an ad hoc basis.

A summary of OD&D ability scores can be found here.