Sure, they all have something in common. They all begin with the same God who kicked Adam and Eve out of the Garden of Eden, told Noah to build a boat, and made a covenant with Abraham. They all accept the story where Judah Ben Hur told the King of Siam to let his people go (or something like that). So, same God, right? I'm not so sure.
According to Jews and Muslims, God is one and indivisible. According to most Christians, God came down in human form as Jesus Christ, and he is split into the Trinity, giving you three gods for the price of one. (Christians will insist that they have a monotheism, but honestly I think that the Muslims and Jews make more sense when they think of that concept as being decidedly polytheistic.) So, the very nature of this same God they believe in is different. If I told you that I owned a certain kind of animal that was a mammal, and you said that you had the same one but it was a reptile, would you think that's the same animal? (That might not be the best analogy. Work with me here. The point is, how can a Trinitarian God and an indivisible one be the same thing?)
Perhaps you think that's a nitpick, but we can go on from there. Jews don't even recognize Jesus at all, but for Christians, he's easily the most important part of the whole concept. The sacrifice of Jesus was necessary for salvation and what everything in the Jewish scriptures was leading up to. Jews don't even believe that humanity needs the kind of salvation that Christianity claims that we do. Plus, tehy can give you an entirely different interpretation of their own scriptures. (But hey, what do Jews know about the Torah anyway?) So, supposedly it's the same God who has two completely different concerns regarding what his creation needs. How is this the same being?
The difference between Christianity and Islam is similar, as Muslims absolutely reject the idea that God came down in the form of a man and "died". They also do not accept that God can be split into a Trinity. Unlike Jews, they do believe that Jesus was a prophet and he plays an important part in their overall theology, but to say that a guy's not God is saying something drastically different than saying that he is God.
I'm not denying the historical link between all three of these religions, but they each define their deity in such different ways that it's like they're not even talking about the same thing. Even when you start to break it down by denominations/sects/schools of the three main faiths, you still wind up with a singular God who's described in so many different ways that it's hard to imagine that they're all talking about the same being. Are the Orthodox Jews correct and the creator of the world gives a crap about mixed fabrics? Seems like a different sort of personality from the being of Reform Jews who is more hip with changing times. One way or the other, he certainly can't be both.
Of course, Christians aren't off the hook. We can go as extreme as Mormonism (which is arguably more different from mainstream Christianity than Islam or Judaism when you've got a God who used to live on the planet Kolob.) And it sure doesn't seem like Methodists are talking about the same guy that the Westboro Baptist Church rant on about. Same name? Yeah. Same being? I can't find my way to that conclusion.
Even in the earliest days of Christianity, there were strikingly different views as to what Jesus even was. Some considered him to be fully God. Some considered him to be more along the lines of an "adopted" son. It's like they weren't even talking about the same thing, just many different ideas that they all wanted to call Jesus.
Despite the fact that all these Gods stem from the same source, there is reason to believe that this is hardly a unique phenomenon. The Norse goddesses, Frigga and Freyja, are considered different goddesses, but there is reason to believe that they might have evolved from one singular goddess. With enough time and distance between related peoples, its not so strange to have that sort of a thing happen. You'll also find similarities in gods from different cultures. For instance, the Slavic god Perkunas shares many of the same attributes as the Norse Thor, as they both control the thunder and lightning while riding goat-pulled chariots in the sky. It's not so unlikely that the two descended from a common ancestor, if you'll pardon the evolutionary wording of that statement. (It should be noted that they have a lot of differences as well - but I'd argue that their differences are no greater than the differences you'll find within the Abrahamic faiths.)
I realize that as of this point, I've probably completely lost most theists, as I'm treating Yahweh/Jesus/Allah the same as any other mythological deity. But if you're with me so far, then consider that there has been at least one study which shows that people who believe in God generally tend to think that God agrees with them on important issues. It's pretty rare to meet two theists who see God in exactly the same way, but even when they're close, they'll ultimately be describing somebody who's more their idealized self than anything else. I've noted this before when it comes to Jesus, as people (even some atheists and agnostics) will talk about how wonderful he is, but they're essentially describing a character that they've invented in their heads, only borrowing the bits of the New Testament that they like.
I realize that it's a very inclusive, accommodating point of view that creates this idea that all of these people are worshiping the same being. Some will take it even further and say that basically any idea, be it Hindu, Sikh, Zoroastrian, deist, etc. of a deity essentially amounts to the same thing. While the motivation might be a good one, it ultimately trivializes one hell of a lot of history and a great deal of cultural diversity.
I once debated a Christian and I spoke of how there are other gods, but the Christian kept insisting that there is only one God. I tried to make it clear that while I understood that, according to his belief system, there is only one God, there have been thousands of gods that have been worshiped all over the world for a long, long time. He wasn't having any of it. But honestly, now I'm not even sure that I agree with as much as I was willing to grant him. Yeah, people might be using that generic term "God" and think that they're all talking about the same thing, but I can make an argument for many different monotheisms - perhaps even one for each theist out there.