Actually, I'm of the opinion that a gift implies that they did you a big favor, rather than the fact that they just told the truth. Yes, they took a few minutes on the phone or at the computer to write a letter, but presumably you thanked each one at the time they agreed to serve as a reference. Sending them something now actually implies that they exerted undue influence, that you didn't really deserve the job offer.
I've never gotten a gift from anyone for whom I gave a reference, and I've never wanted one or expected one. It almost seems like a payoff, as if I would only give references to increase my stash of gifts. If you really are determined that you must do something, then drop each a brief email (or a personal note if you insist) after you have been on the new job for a few weeks, saying how much you enjoy it and how you appreciate their reference that added to your portfolio of qualifications.
These people have done this as a part of their professional lives. They don't get showered with gifts for everything else they do on their jobs, and they shouldn't for this.
What you do is pay it forward - you offer references for qualified (that's essential) applicants in the future, and if the situation were ever reversed and you could serve as a reference for any of the 3, then you offer that.