1. I understand the establishment clause as the two related freedoms of religion. The first one protects people from having a set of religious beliefs imposed on them by the federal government. Second, people are protected from having their own religious beliefs restricted or threatened by the government. The government is also forbidden from favoring one set of religious beliefs over others or favoring any religion over non-religion. This was founded based on the diversity of religious beliefs that many settlers from Europe came with during the early United States. The lemon test was designed to decide whether a law or government action might promote a particular religious practice should be allowed to stand. The criteria for a law or action to be constitutional is that it must not lead to excessive government entanglement with religion. It cannot either inhibit or advance religious practice. And there must be some non-religious justification for it. 

2. Burning the US Flag is protected by the first amendment. During the Texas v. Johnson case, the supreme court decided that burning the US Flag was a form of symbolic speech that the first amendment protects. The flag desecration law was found unconstitutional to uphold as it restricted this kind of freedom of expression of burning the flag at the same time. In response Congress has tried to overrule this court decision with various flag protection acts to re-criminalize the act but to no success.     

3. Taking the fifth means protecting yourself against self incrimination or the right to remain silent. People have the right to not to give evidence in court or to law enforcement officers that might constitute an admission of guilt or responsibility of the supposed crime one is being accused of or involved in. It is also embedded that suspects are required to be informed of their most important rights, including the right against self incrimination before being interrogated in police custody. However for law enforcement it is not necessary to inform suspects of their rights before they are questioned in situations where they are free to leave. It is a constitutional right.