I agree though that Walker and Fredette aren't in the same league as the other two but I think you would be amazed at the amount of people who would think otherwise.
Big scoring numbers and media attention go a long way.  
I will say though that while Fredette has looked like a pure volume scorer, Walker has a much broader skillset and can play pretty good D at times too.  I think he'll have to rein in his offensive game a lot to fit in on a good team, but he'll be a NBA starter at some point.
I am not so sure with Walker.  I don't think he is a good enough shooter or floor leader to make it as an undersized PG in the NBA.
I mean, I don't think he does anything better than Nate Robinson for example, and is reportedly, not much bigger (it will be interesting to see actual measurements).
I wouldn't dismiss Walker by any means, I could see him being a very effective two guard. Maybe a smaller, faster version of Ben Gordon. 
The problem is, Ben Gordon is a great shooter.  Walker is average at best.  Plus, Gordon is probably 4-5 inches taller than Walker (who I have heard is closer to 5'9" or 5'10")...that is significant.  
If we were talking about an early second round pick, I would be all over Walker, because you can't teach heart, however, as a lottery pick, I would be beyond disappointed if my team took him.  
I think Singler is going to be the best out what we saw last night. 
I like Singler as a late first round sleeper.  He will be a quality role player in the NBA.  But, I don't think he will be anywhere near as good as Irving and Williams.