I am considering to buy this game, but there is a little problem: we are just a small group (3 people) Can we play the campaingn or adventures from FFG with only 2 PCs?

Without problem.

Most of them are oriented for 3 to 4 players. You will need to scale down combats a bit, that is all.

2 PCs isd enough for most, yes. the core set is designed for 3 PCs and one GM, so you can even find anohter one if you want

i never played with 2 though, but it surely works.

and never have more than 5, it gets WIERD then

Yup 3-4 is the sweet spot. 3 is doable keeping any eye on combat #'s etc. (really you have to customize them a bit anyway since groups vary so much).

If it's 2 PC's, that gets a bit trickier but is still doable. Works best if the social/investigative aspect of game is emphasized more then (essentially strip away more of the D&D side and reveal more of the Call of Cthulhu truth lying beneath it, bwah hah hah, leave them insane not dead).

I was GM for two players, it was great. There was really good roleplaying between the players partly as a result of them beeing "only" two and thus needing to work together more to overcome obstacles.

So it works really well for sure, I can recommend it warmly.