This is a 3-Player FFA game. Player 1 plays and activates the ability of Mindslaver, targeting Player 3 and ends his turn. Player 2 takes his turn, doing nothing important to this matter. As Player 3 starts her turn, she hands her Hand to Player 1, and Player 1 draws for her. If Player 1 were to play Venser Sojourner's (on Player 3's field already) +2 ability, would Player 1 be able to target his own cards as well, or only Player 3's?
Player 1 controls the decision-making of player 3, but nothing else. Any effects that player 3 plays that refers to stuff they own or control only refers to stuff player 3 owns. Likewise, anything they cast that only affects stuff they don't own/control can be used to target stuff player 1 owns (stuff like an overloaded vandalblast would still blow up all of player 1's artifacts, for example). Player 1 might be the puppet-master for a turn, but that doesn't change who owns or controls what. So Venser referring to 'permanents you own' only refers to permanents player 3 owns.
Thanks and sorry for the convoluted wording.
715.3. Only control of the player changes. All objects are controlled by their normal controllers. (…)
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules