Someone had to dig a large hole in a busy road and gingerly plant a massive bomb then conceal the whole thing, run the det wire to a nearby Palestinian building, then man the thing for hours until the convoy came by, detonate it and exfiltrate without getting caught. In Gaza. In full view of a PA checkpoint. Surrounded by milling Palestinians.
Quite a coup for an Israeli bystander, eh? Even if it were a Palestinian working secretly for the Mossad, the things the Palestinians do to spies and traitors (even rumored and suspected traitors) are sufficiently ghastly to make this very unlikely.
Would the Mossad be willing to waste 3 American security personnel if it meant US blessings and free reign to deal with their situation? Possibly. But they don't need to.
Much more likely that Hamas or Islamic Jihad did this deliberately to wreck and drive off international intervention in their single-minded pursuit of the destruction of Israel. Or maybe the bomb was set up to get someone else, and the convoy was just a target of opportunity that happened along. I'm betting that whoever did this definitely prays to Mecca 5 times a day.
What is known is that Arafat personally ordered the execution of the US ambassador to the Sudan, and Palie terrorists have killed dozens of Americans for decades, including many of the recent terror victims in Israel.