NCAA throw-in violation
 

Art. 2. No player other than the thrower-in shall:
a. Perform the throw-in or be out of bounds after a designated-spot
throw-in begins.
b. Be out of bounds when he touches the ball after it has crossed the
vertical inside plane of the boundary line. Repeated infractions shall
result in a CLASS B technical foul.

If all four other team mates are in bounds when the throw-in begins, but one of them steps on the line as they are running down the sideline, and is back in bounds before the ball crossed the vertical inside plane of the boundary line and caught the ball. Is this a violation?

Not under this rule. It might be a violation for something else. But there are other requirements for something else to be a violation for a player being out of bounds in your question.

Peace

Section 3. Player Out of Bounds
Art. 1. A player who steps out of bounds under his own volition and then
becomes the first player to touch the ball after returning to the playing court
has committed a violation.

My first post is Section 4, both under Rule 9. I was told by the other ref that Section 3 also applies on throw-ins as well. I figured that since there is a separate section for throw-in violations, that Section 3 only applied to live ball situations. Thoughts?

A throw-in is a live ball situation, though.

No it's not, or at least no one is going to call it who wants to work above the middle school level.

Agreed. There's an element of "near the throw-in spot" or "on the same boundary line" or "confuse the defense into thinking it's not a spot throw-in" involved in the rule.