1. Is it mandatory to worship any deity in Deism?
No, mandatory worship is not a common belief in deism, as it is generally believed that there are no consequences for refusing to do so. However, if you wish to worship God, you may certainly do so. I do at times... just not within the walls of a church or temple.
2. Why can't God be involved with everything that happens in this universe? Or is this concept not general?
That really depends on the individual deist's theological views. Some believe he/she/it physically cannot intervene, and others believe he/she/it can, but chooses not to. Personally, I believe that the laws of science and nature were created by God. God then stepped back away from the world, and did not intervene any further. It let those laws govern the world instead. However, I, myself, am unsure as to whether or not it is able to make the decision to intervene or to not intervene.
3. Finally I want to know which religions fall under the category of Deism?
Deism is a philosophy, more or less, but it could also be a religion in and of itself. To me it is both a religion and a philosophy, however, deism as a philosophy can be adhered to by any religion. There can be Christian deists, Jewish deists, Muslim deists, and pretty much any other religion as well. In essence, deism is simply the belief in a supreme being that is distant from the universe it has created. If you can manage to fit that sort of ideology into your personal religion, then voila, you are both a deist, and a practitioner of that religion.