No sane umpire is going to call the third baseman for being in foul ground if he's straddling the bag naturally. If a coach points it out, umpires will enforce it begrudglingly.
As to obstruction - merely being between the runner and the base is not obstruction. That rule is a lot more involved, and I suggest you read it in entirety ... trying to explain it here without that background reading would not be productive ... it would lead to 92 what-ifs from various people wanting to prove how smart they are.
