Your major league team carries two guys who can play third base for you. One of them is old, has an OPS+ of 57, no errors and absolutely no range. The other is not old, has an OPS+ of 72, has mediocre range and a fielding percentage barely above .900.
You perceive, because you are smart, that this not only sucks but swallows as well. Here's how to get the taste out of your mouth...
One of the few Giants farm-hands hitting well this year is "Nasty" Nate Schierholtz, whose surname translates from an obscure German dialect as "WTF? I'm blocked by Randy freakin' Winn?" or something like that. Keep in mind that in Giants-speak "hitting well" means "doing something more that slapping weak grounders back to the pitcher",
Why not give him a chance to settle the situation with the big club and have him man third base for us this year? Although he's been a right fielder for the past three minor league seasons he was drafted as 3B. His range is no worse than Aurilia's and his fielding is no worse than Castillo's. He put up an 85 OPS+ in limited duty with the Giants in 2007.
There's no argument that long-term his place is in the outfield, almost certainly right field, but he'd be just as good a choice as either of the others for this season. Any mistakes he makes as an infielder won't carry over to his development as an outfielder and it may be time to see if he's ready to hit major league pitching.
Let Aurilia take the starts against tough lefties, dump Castillo by any means necessary (he's cheap to simply DFA and lose on waivers) and see what Nasty Nate can do at the hot corner. He's got upside that the other possibilities lack and since he's an outfielder provides lineup flexibility that the other two cannot.