i just had the exact dilema described here as I was drawn third in my draft and was debating the issues of taking either one of the aforementioned 3 RBs.
Alexander - If you minus last year, he is one of the most productive RBs year in year out. Personally, I beleive his foot is 100% and with Hasselbeck being fully healthy too, there should be a nice pass/run balance once again. In addition, I think their WRs will be even better this year with Branch, Hackett and Engram working the slot and not forgetting Burleson as well.
LJ - The main thing that bothers everyone has to be talks of a holdout because if it was certain he is going to play next year, he would be the concensus number 3 pick and perhaps number 2. However, lets break down the issues surrounding him. Losing 2 key lineman in the last 2 years has got to damage his production no matter what, but losing Green isn't as bad. Green was out for most of last year and Huard stepped in but LJ's numbers were fine and the addition of Bowe should improve the pass game too. At this moment, I believe taking LJ is too risky with talks of a holdout.
Gore - Until last year, he was a relative unknown but he exploded onto the scene and carried many teams to championships. He had his fumbling problems but by the end of last season he had that under control. However, losing Norv Turner will hurt but I believe the development of Smith, Davis and the acquisition of DJax should offset this.
In closing I believe LJ the High rish high reward pick regarding his contract situation so if you want to take a flyer, pick him. Many have dismissed SA as being too old and maybe even still injured but I have a gut feeling that he will produce this year. I think Gore is the safest pick out of the three as he hasn't got any contract, age or injury concerns lingering over him.
P.S. In my draft the guy at 2 took LJ and SJax fell to me so you could always hope for that!