Need to make contact for obstruction?

No, there does not have to be contact to be obstruction. If a fielder's physical presence without the ball hinders the runner, it is obstruction. In your case, if the runner turned to avoid the third baseman in his path, that would be obstruction.

 Not sure I would call obstruction because the runner turned around.  In a run down, I am not a mind reader, and with the runner going back and forth how can an ump know why he reversed direction.  And how can you be sure the 3rd baseman would not have stepped to the side to let him by?    I agree, obstruction need not involve contact, but the runner would need to be hindered in his attept to reach the base by trying to avoid contact.   I don't see it in a run down as described.   Now if he was being chased by the 3rd baseman who had the ball and had to run around a player that did not have the ball, then yes. 

Obstruction.  Contact is absolutely not necessary for obstruction.  We don't have to know why he turned, we just have to know the F-5 was blocking 3rd base when R-3 was trying to return and R-3 turned around. In this case, I would give the benefit of the doubt to the runner who was confronted with a teammate already occupying 3rd base and F-5 illegally blocking the baseline. 

Definition of obstruction includes: 

"when a fielder . . . hinders a runner or changes the pattern of play . . . "

and 

"fielder without possession of the ball denies access to the base the runner is attempting to achieve."

Umpire the game in front of you, not the what-ifs. A play like this is always in the eye of the beholder, but don't go down the path of what may or may not have happened. You should be able to tell if the runner turned because of F5 or if he was doing it independently.