When a monstor is immune to, lets say +3 weapons and lower, does it mean weapons with lower than +3 thac0 or the additional damage?

The weapon has to be +3 or more to make any damage at all. This has nothing to do with you characters base THACO.

I might see how this is confusing because some weapons, and i believe flails fall in this category, have a +1 even when they are non magical weapons. When it says that a monster must be hit by a +3 weapon, it means the ThAC0. Additionel damage such as the one from the flails or elemental damage doesn't count. Allthough said damage might still come through. For instance if you have a weapon that does elemental damage like fire damage and you are attacking a monster wich isn't immune to fire damage will still take that damage even if the weapon isn't of a high enough magickal status to hit said monster. I hope that made sense.

JR

Damned i lost cousin Jan's techno gloves