Well there is absolutely no reason why you can't learn Java without knowing the other two. It would require more effort, but nevertheless is still easier. In fact, learning C/C++ is much more difficult.
No wonder nowadays, there is an increasing tendency to directly learn Java. But it there a problem in this? I always felt so.
Learning OOPS concepts through Java is possible; but do you think you can visualize the concepts to the extent you can with C++. Also basic CS stuff like Data Structures, Searching, Sorting is better understood through these languages than Java.
When I read Joel On Software's post about this, it made 100% sense to me. You can read it at:
http://www.joelonsoftware.com/articles/ThePerilsofJavaSchools.html
