John Gill Commentary 1 Chronicles 2:3

"The sons of Judah: Er, and Onan, and Shelah; which three were born unto him of Shua`s daughter the Canaanitess. And Er, Judah`s first-born, was wicked in the sight of Jehovah; and he slew him." — 1 Chronicles 2:3 (ASV)

The sons of Judah
The genealogy begins with him, though the fourth son of Jacob; because, as Kimchi says, this book treats chiefly of the kings of Judah; but rather not only because Jesse and David sprang from him, but also the King Messiah:

Er, and Onan, and Shelah, which three were born unto him of the
daughter of Shua the Canaanitess ;
see (Genesis 38:2Genesis 38:12) ,

and Er, the firstborn of Judah, was evil in the sight of the Lord ;
because he did that which was evil:

and he slew him ;
he died by the immediate hand of God, and so his brother Onan, being wicked also, (Genesis 38:7Genesis 38:10) .