This book was a fantasy book based in a medieval time period with magic. The start of it was the main character as an old man who was a retired spy/assassin for a king reminiscing on his past adventures and accomplishments. He was a stable boy or servant and somehow came into possession of a magic stone/crystal that granted him a power(I believe this one was invisibility). I think there were a finite amount of these stones and each had different powers and I think he needed to make an agreement with the supernatural beings that the stones draw their powers from. The stones drew their power from creatures from another realm/world and I think the cost of using them was pretty high, possibly his soul or something along those lines. One of his first exploits was as a page to a knight and he was in the king's army assaulting a castle, he used the stone to turn invisible and infiltrate the gatehouse to unlock the gate to the castle and let the army in. It was probably about 10 years ago when I read it, but I believe it was a young adult fantasy book or just a normal one.
Cycle of Fire by Janny Wurts perhaps?
It does involve a world with medieval culture, magic crystals, and three child protagonists.
The first novel in the The Circle of Fire trilogy. As a new plot threatens mankind's survival, three children stand at the crux. All possess extraordinary talents, but each of them is flawed. In need of all their help is the Stormwarden, last Vaere-trained sorcerer, condemned now and trapped in a desperate quandry.