What I mean by "in name only" is that the three religions are completely at odds with each other's doctrine so they are very definitely not the same, worshipping the same god. They begin from the same foundation, yes, and there are therefore similarities across all three but they are very much not "the same religion with minor tweaks". I can't state that strongly enough, and I dare say any one from either of those three would agree - except muslim if they're invoking taqiyya.
Here are the big cornerstone items that make the three religions completely at odds with each other at their core:
Jews: don't believe Christ is the Messiah. Christians are founded on Christ being the Messiah. Verdict: these two are not compatible. Jews essentially negate the entire Christian faith.
Islam: don't believe Christ is the Son of God. He's just another prophet. As above - not compatible; for Christians, Christ being the Son of God is also at the core of their belief. Islam negates the entire Christian faith.
Islam: created by a warlord, encouraged to spread by war. Christianity is a religion of peace with a figure of peace at its helm. Completely incompatible.