In a nutshell, Valentin should hit the most homers, either he or Cabrera will give you the most rbi's, runs is a toss-up and Furcal has the highest runs/stolen bases potential. None of them is worlds above the others at this point, and all are off to weak starts.
That said, I'd rather have Cabrera than Valentin. The extra homers Valentin will give you aren't worth the average and steals you'd give up. Cabrera is several years younger, as well, so there's a chance he'll still improve a bit. I'd rather have a healthy Furcal who plays every day than either of the other two, but that may well be wishful thinking at this point.
If you're set for power and play Cabrera more than Valentin anyway, you can consider taking a risk, dropping Valentin for Furcal and hoping he gets over his early jitters. If Valentin works out better than Cabrera on your team, I'd stand pat, at least until Furcal shows signs of warming up. If I had to make the decision, I'd probably grab Furcal, but I wouldn't be too confident I was making the right choice.