Art. 4. For men, a maximum of six players (four opponents of the freethrower and two teammates of the free-thrower) shall be permitted on the free-throw lane during a free throw. All other players shall be behind the free-throw line extended and behind the three-point field-goal line.
a. (Men) Within this limit, opponents of the free-thrower may occupy the third lane space (with the spot closest to the end line being the first). If they opt not to, a teammate of the free-thrower may occupy the third space. No player shall occupy the fourth space.

Can, at any time, a third member of the free-thrower's team occupy the third lane space if the defensive team opts not to use it?

no, only 2 team mates of the free thrower can be on the line, they are in the 2nd lane space unless one of the other defenders do not want the 3rd lane space, then they can move up one, but the bottom 1st lane space has to be filled with 2 defenders...

No. Maximum of 2 allowed- no matter where they line up on the lane.

So in High School it is designated lane spaces which means they cannot have the third space if not occupied.

In college the defenders can have the third space if not occupied (max 2 defenders)? (Womens & Mens)

Is this correct?

first off
 

MAX 2 for the offensive team and MAX 4 for the defensive team. The only restriction is that the bottom 2 spots MUST BE DEFENSE -- the other 4 are open -- OFFENSE HAS rights to lane 2 and Defense has rights to lane 3 -- if for any reason defense only uses 3 players the offense can take the 4th open space.

It's exactly the opposite of what you've posted for FED and NCAAW. In FED, either the offense or the defense can take the unoccupied spots (ignoring the lowest spot, below the block, of course), within the constraints of the number of people allowed. In NCAAW, the teams can only occupy (or not) the spaces to which they are "assigned."