What experiences in ch. 3 of The Pearl make Kino realize that his innocence has put him at the mercy of other people?
After finding the pearl, Kino and Juana are excited about their future possibilities. Their neighbors in both the town and the village are all talking about their good fortune, and many are dreaming of what they would do if they found such a pearl. Some, however, are not just dreaming but scheming - and these take action to try to get their hands on some of Kino's newfound wealth.
Kino realizes his innocence puts him at a disadvantage when he does not have the knowledge, wisdom, or experience to challenge the claims of either the priest or the doctor. The priest is the first to visit; he flatters Kino, telling him he is "named after a great man - and a great Father of the church." Because Kino does not know how to read and does not know church history, he does not know if this is true or not - he is powerless to respond, and can only hope that his son will become educated so he will not be at such a disadvantage:
Some day, his mind said, that boy would know what things were in the books and what things were not. The music had gone out of Kino's head, but now, thinly, slowly, the melody of the morning, the music of evil, of the enemy, sounded, but it was faint and weak.
The priest goes on to encourage Kino and Juana to give generously to the church to show their gratitude for the pearl. Kino does not reply to this, but Juana says they will and also lets him know that Kino has said they will to be married in the church.
The "music of the enemy" grows louder when the doctor appears - the same doctor who turned Kino and Juana away in an earlier chapter because they did not have the money to pay him. Now, the doctor says he was "out" before, but he is here now to check on Coyotito. When Kino tells the doctor that Coyotito is now nearly well, the doctor replies:
"Sometimes, my friend, the scorpion sting has a curious effect. There will be apparent improvement, and then without warning - pouf!" He pursed his lips and made a little explosion to show how quick it could be, and he shifted his small black doctor's bag about so that the light of the lamp fell upon it, for he knew that Kino's race love the tools of any craft and trust them.
The doctor suggests it may only look like the scorpion sting is healing, when in fact it is getting worse. This does not sound right to Kino, but since he doesn't know for certain, he is at the mercy of the doctor. He and Juana both want their son to be well, so they let the doctor treat him by giving him a capsule of white power. Soon after this, Coyotito becomes ill, and the doctor must give Coyotito more "medicine" to make him well again.
When the doctor asks for payment, pretending not to know about the pearl, Kino's eyes accidentally drift to the place he has hidden the pearl. He tells the doctor he will have to pay him later - but not trusting the doctor, he buries the pearl in a new location. This proves to be a good decision since someone breaks into his hut to look for the pearl in the spot he had buried it before.
Both of these instances, as well as his experience with the pearl buyers in chapter 4, show Kino the disadvantages of "innocence" and a lack of education. This makes him all the more determined for life to be different for his son.