In the old days, refs were responsible for seeing to it that there were 3 feet between the inbounder and the defender. Sometimes they would actually call out "3 feet!" to both players before giving the ball to the inbounder. This was implemented by having the inbounder give a 3 foot cushion to the baseline, not by the defender backing up. The defender could step right up to the line. So the defender would be in violation of the 3-foot rule if he/she crossed the line out of bounds. However, the 3-foot cushion--like the 5-second rule for inbounding or the 3-second rule in the lane--is squishy. It's not built on precision. It's just the ref's judgment. So unless you are noticably crowding the the inbounder by moving over the baseline I doubt any ref would call it. If they did, I believe they would first blow the whistle to stop play, then remind the defender to give 3 feet and give the ball to the inbounder again, with a fresh 5 seconds.
Besides not being able to touch the inbounder, the defender also may not touch the ball unless/until it has left the inbonder's hands. Touching the ball resulted in a technical. Guess touching the inbounder would, too? But I do not recall.
I don't think that the 3-foot rule was always implemented as stringently on other out-of-bounds plays, like on the sideline. I do remember it for sure on the baseline after a made basket. I think you could play a little tighter on sideline inbounds plays. It would make sense to have it still be 3-feet but I just dont remember it being enforced quite the same way.
[These were high school rules in Iowa when I was growing up so they may or may not match up with the rules in MCBB or WCBB.]