Albert Barnes Commentary 1 Chronicles 3:17

"And the sons of Jeconiah, the captive: Shealtiel his son," — 1 Chronicles 3:17 (ASV)

Assir - The name Assir suggests he was perhaps born during the captivity. He may have either died young or been made a eunuch (Isaiah 39:7). After Assir’s death or mutilation, the line of Solomon became extinct. According to the principles of Jewish law (Numbers 27:8–11), the inheritance then passed to the next of kin: Salathiel and his brothers, who were descendants of David through the line of Nathan.

When Luke calls Salathiel “the son of Neri” (Luke 3:27), he is giving his real, or natural, descent, since no genealogy would assign an inferior and private parentage to the true son and heir of a king. Therefore, not only Salathiel but also his brothers, such as Malchiram, were all reckoned as sons of Jeconiah.