You can find Dragon Scales in one of the houses near the market at chapter 3.
Is this true? This means you don't have to bother siding with anyone. Which house are you talking about? most houses can't be accessed in chapter 3
And that's Roche quest? And just FYI to you
that the sword from gargoyle quest isn't even near as good when doing magic based build
Nope, the dagger quest basically depends on whether you choose to help Philippa escape the dungeon or get the Nilfgardian ambassador to lead you to Triss. If you choose to go after Triss (that will escape whether you free her personally or not), then you can't get the dagger (and I'm assuming you won't be able to get the scale either).
You CAN get the dagger even if you go with Triss and dragon later. When you defeat the dragon you will come back to town and see Iorveth getting captured by some scums. Defeat them and you will get the dagger. However, you won't be able to lift the curse in that play through, you just get the dagger, meaning Saskia's curse can b e lifted even if you go for Triss, contrary to the choice presented. (just not in this game and this path, but the fact is you still have dagger for her).