I now have three runes, FINALLY! Just started Chapter IV. I always figured my goal would be 3 Earth Runes ever since getting the first from Zoltan.

But now I have 2 Earth Runes + 1 Moon Rune. Ends up being 30% more damage + 15% higher silver sensitivity for monsters. I figured that would equate to +45% Damage?

3 Earths would be 45% damage. What the hell's the difference? Aren't all monsters sensitive to silver? The mechanics in the first Witcher game can be so damn confusing. I think they're lacking some clarity.

According to the chart on the wiki, three Earth Runes is most expensive. Highest sell and buy value. But which one is actually most useful? TO ME, in my mind, the 2 Earths + 1 Moon Rune makes sense. But the triple Earth Combo is making me torn.

Two Earth and one Moon is a great combination because you get the damage, the sensitivity to silver and 75% chance of Knockdown. Your assumption that all monsters are sensitive to silver is wrong. Some creatures are more sensitive than others and even some (though few) that are not sensitive at all. There are even a few that are sensitive to steel. So, yeah, 2Ex1M is a great combo.