There were more than three. The rest of them took off after the original hit and run raiders, leaving the three at the checkpoint. Whereupon the second wave hit. It's called a feigned withdrawal, and it's literally one of the oldest tricks in the book--it was ancient when the Romans used it. And it still works, unless your troops have iron discipline.
Exactly. The bulk of the patrol took the bait, but the ambush wasn't for them, it was for the security element left behind. No doubt the insurgents have seen this phenomenon before, and realized "Hey, we could probably sneak up on the small team that's left behind and bag them".
As far as that goes, it shows either criminal or strategic thinking. Yousefiyah itself is a farming area near Baghdad rife with kidnap for ransom gangs and Sunni extremist sympathizers. It's very likely that local insurgents/criminals caught these guys to sell to al-Qa'ida in Iraq or some other major group. It's possible that a larger group just did it themselves, but it seems to be a local job by people who were very, very familiar with the terrain, and the specific tactics used by these checkpoint guards.
Either way, it would have been a lot safer to gun these guys down and shoot a quick propaganda video rather than take them alive. They're taking a huge risk doing this, and that leads me to believe that this wasn't just some clever insurgents who pulled off a nice ambush. It was a coordinated maneuver to take hostages by people with a plan.