I do believe that Sikhs, Jains, and Buddhists can be Hindus, but not that they have to be. Hinduism is the creator which sprouted all of these other religions, but a Buddhist may not want to be a Hindu, or a Jain may disagree with some of Hinduism's teaching, as well as a Sikh may say the same.
But I personally do see myself as more than one religion. I am a Hindu (firstly) and Sikh as well. I wear the Sikh kara and don't cut the hair on my head.