There have been two major effects of these amendments.
First of all, all three of the amendments taken together have been used to advance the cause of minority rights in the United States. This is especially true of the 14th Amendment. This amendment says that all people shall enjoy the equal protection of the laws. This clause has been the basis for much of the movement towards equal rights for blacks, women, gay people, etc. The 15th Amendment has been used to protect the voting rights of minorities from being diluted either explicitly or through more circuitous means.
Second, the 14th Amendment has been used to incorporate the Bill of Rights, making it apply to the states as well as to the federal government. Until the early 1900s, the Bill of Rights did not apply to state governments. Over time, the Supreme Court has used the 14th Amendment to require states to abide by the Bill of Rights as well.