I'm brand new to the game and I read in the rulebook that you can only have one copy of a plot card in your plot deck. Quite often, though, you can find three copies of a plot card in a chapter pack. Is this just a side effect of the 3 per pack collation or are there ways (or other cards) to add more to your plot deck? I'm guessing the first, since Valar Morghulis specifically says there can be only one in your plot deck.

This is because there are 3 copies of each card, including Unique and plots.

You may have several copies of a single plot in your plot deck only if allowed by the card text (such as Power of Faith, Power of Arms…)

3 copies of a plot is often usefull when you build several decks at the same time and don't want to use proxies.