Where can i find those items in Chapter 3???? (Harpy egg and rotfiend's tongue???)

Those are supposed to be quest items i beliave....

Rotfiends can be found in the sewers and Harpies can be found near the entrance to the city area where you first came in.

You're probably referring to the Queen Endrega's pheromone. This game has it's flaws but damn do the positive's outweigh the negatives(like finding out you need random Chapt 2 items in Chapt 3).
Funny thing is that once upon a time in Act 2 I saw a Harpy Egg and left it on the ground at the quarry...or if I did get it I didn't save... : '(

I made the same mistake, but things like this make sure, that you will play it through more than once.
Just like TW1. Even a complete printable shopping list can be found for it.
TW2 goes much easier on you. :)

Starting at the market area, if you go past the entrance to the Nilfgaardian quarter, you can find a slope of rubble that you can climb. Up there is an infinite supply of harpies (i.e. they respawn after a day or so). Depending on your story choices, this might also be the only way you can take to reach the market in the first place, so you may already know what area I'm talking about. As for the rotfiend tongue, they usually spawn in the sewers - the entrance is again near the Nilfgaardian quarter, if you instead look to the right where the Mighty Numa hangs out; not far away from him and Marcus, there is a door leading down into the sewers. The enemies you can meet in the first room are either bullvores or rotfiends, and they respawn just like the harpies.

Some of the other items you cannot find past a certain point in the game (such as the queen endrega pheromones), but these items are marked as 'quest items,' so I was too scared to sell them anyway, the first time I played. This isn't too harsh, in my opinion. Like Rhonmeh wrote, this game goes much easier on you than TW1 did.