Amateur profile at work: Not a professional!
Reaching way back to my profiling and behavioral psych classes, I remember that as the age of the victim goes up, the age of the offender goes down. So, that would mean that the offender is likely under thirty, I would say probably 21-28. This does not take into account things that might stunt a person's developmental age, like time in jail, or extreme sheltering by a parent. So, he could be physically as old as 40, but developmentally in the mid-20's.
These are high risk victims, and there doesn't seem to be any real attempt to obscure their identities, this does not seem to me to be an organized killer. Just because he hasn't been caught doesn't mean he's good at it, just that he is lucky and knows that the police are overburdened. he has been caught on surveillance, so personally, this guy isn't that great or experienced a killer.
He is killing at a very fast pace, especially if these three murders are his first three. I don't know what that means, but it could indicate that he has been incarcerated or otherwise indisposed for a while, and this need has been building with him. It is likely that the people close to him, if there are any, at least suspect that he is capable of this. It's likely he's been talking about this, or something like this, for at least a little while. He might have stopped talking about it now that he is able to actually do it.
He likely lives or works within the triangle formed by the murder/dump sites.
I find it interesting that the "retail store" is actually a charity store that donates it's proceeds to charity. That might be coincidence, but it could also be a message. this guy is killing homeless men, one right outside a store that aims to help the homeless and others.
The dates of the murders might give clues as well. Two on Wednesday/Tuesday night, a week apart, and then one on the following Saturday/Friday night. The last would have been New Year's Eve, meaning that it is not likely this guy has friends or family, at least not any that were any interested in spending the holiday with him. I think that the lack of close personal relationships is backed up by the nature of the crimes. He's stabbing people, that's up close, personal, dirty work. That's not something that you can really explain away when you come home soaked in blood. He likely lives alone or with someone whose work schedule or physical condition makes it easier for him to come and go undetected.
That's about all I got, but I wish I could figure out a little more.