Like @Harel13 notes there are differing opinions:
The Malbim spells out the differing viewpoints:
- There are those who say that "Assir" is descriptive of Yechonya since he was incarcerated in prison and there he gave birth to these sons.
- The opinion of the Sages is that Assir and Shealtiel were his sons; he had two sons, and Malchiram and Pedaya are the sons of Shealtiel.
- Alternatively, all of them were the sons of Yechonya.
The Metzudos Dovid says that Shealtiel was the son of Assir.
Ralbag says Assir was the son of Yechonya and Shealtiel was the son of Assir. Then, when it mentions Malchiram and Pedaya, they were the sons of Shealtiel.
Finally, Radak says that both Shealtiel and Assir were Yechonya's sons. And Assir was so called because he was born in prison.