Muslims believe god gave one set of scriptuers to everyone, but they were corrupted over time. They regard Jews and Christians as trying to follow the true path god set out, but they are handicapped by the corruption that slipped into their scriptures and religion.
They believe Mohamed set things back on the true course god originally ordained.
So, they see all Peope of the Book as trying to follow the true path. In this sense, they see Islam, Christianity, and Judaism as the same. There were also other People of the Book (I think 27 in all) but the other 24 have lost all touch with their original god given religion.
The basic core of Islam is, “There is no god but god and Mohamed is his prophet.” This is reflected in the teaching that god has no partners. That is a rejection of the Trinity and worship of anything but a single god. Jesus is seen as a great prophet whose message was lost and corrupted.
This is a very short description, lacking in many details, but I think it is essentially correct.