3. Boy Seryozha saw two two-headed dragonets, whose heads were tangled. Dragons are either truthful, i.e., both heads always tell the truth, or lying, i.e., both heads always lie. Seryozha decided to help the dragonets untangle their heads. But for this, he needed to know which head belonged to which dragon. He asked the dragonets, and the heads replied:

first: "I am the truthful head";

second: "The third head is my own head";

third: "The second head is not my own head";

fourth: "The third head is lying."

Which heads belong to which dragonets?
The second and third heads contradict each other, which means they are not siblings (sibling heads would either both say they are siblings or both say they are not). Therefore, the third head told the truth (i.e., it is truthful), and the second head lied (i.e., it is deceitful). This means the fourth head lied when it said the third head is deceitful, i.e., it is a sibling of the second head. Therefore, the third head is a sibling of the first.

Answer: The third and first heads are from the same (truthful) dragon, while the second and fourth heads are from another (deceitful) dragon.