Monotheism: the doctrine or belief that there is only one God.
I know very little about Judaism, and even less about Islam, but from my understanding, they do consider Christianity to have shades of polytheism. Or consider it to be outright polytheistic.
This is completely understandable. When one says that a religion is monotheistic, the expectation is that there is one God only, and that God appears as a constant. In Christianity, it's difficult to say that God appears as a constant. There is God the Father, God the Son, and God the Holy Spirit.
Nothing in the definition of monotheism says that a monotheistic God can be one, and yet exist as three others, or ten others, or one hundred others. Granted, there is little that says this can't happen, either. But when you start referring to more than one entity as God, you're usually drifting towards polytheism.
Christianity gets around this problem by bringing out a previously unheard of concept: God existing as one, yet as three persons, all which are unified in will, power, knowledge, and so on. But in a way, Christianity can come across as wanting to have its cake, and eat it, too. It wants to follow the, "Here, Oh Israel, the Lord God is one" and yet keep the Father, Jesus, and the Holy Spirit as God. I can understand why the other two Abrahamic religions would find this polytheism. Given how they use monotheism, there's no room for the concept of the Trinity. If you have two beings with the same type of power and divinity, then you have two gods. That's how it's been throughout history -- at least, from what I've seen. The whole point of monotheism was to start ending the concept of more than one God.
Christianity seems to approach the table saying, "We are monotheistic with the Trinity, and that is acceptable." But is it really? Or is Christianity saying that we have to accept their monotheistic claims on their terms, rather than a set standard, or an objective term? Whereas Islam and Judaism approach the table with their perception of God that matches a set monotheistic standard.
It just seems that Christianity is monotheistic because they say so. For instance, let's turn this around. Say Christians followed a god known as Allah, and Islam had the concept of the Trinity. Can anyone honestly tell me that Christianity would accept Islam as a monotheistic religion? I highly, highly doubt it.
Also, before people start throwing out verses that "prove" the Trinity, I would ask that those people investigate how the concept of the Trinity first came about. There is a reason why there was no official Trinitarian creed until the 4th century. Paul and Peter and the lot didn't suddenly go around saying how God was a Trinity. It's a concept that took time to develop, in order to reconcile the divinity of the Christ with the concept of one God. And even after the Council of Nicaea, it still wasn't happily accepted by everyone. The Trinity is also something that is not clearly or explicitly taught anywhere in the Bible, and I'm pulling this justification from trinitarian scholars themselves.