Well both guys you listed have people in front of them. Fields has to compete w/ Crede he has started to turn the corner and the White Sox love his glove over there. I think he might be destined to switch position or be trade bait for the Sox GM. Kouz has Marte in front of him and even though he has struggled I don't see the Indians letting Kouz pass him by on the depth chart. Also he is 24, soon to be 25 ... it's good that is playing well, but in AA he is pretty old for that league.
I would probably stay put w/ Adenhart, even if you don't like his injury. His stock is going very high now and the kid has a great arm. When it comes to prospects pitching can be risky, though when your allowed to have quite a few (like you seem to have) it's always better to have more IMO. Some will get hurt and never make, but it's hard to tell who exactly. The more options you have the more likely you are to grab an very good young pitcher in the majors.
Anyways you already have 3 options at 3B and though none are really guys with huge upside, I think they are pretty close to Fields overall though (Fields having a bit more power potential). My advice would probably be to try and upgrade at 3B with the talent you have instead of trying to acquire more depth. If you really want one I'd take Fields and would try and drop someone else like Aybar or another prospect you have.