The question requires us to identify which among the given options is not a right provided to a registered Trade Union under Chapter III of the Trade Unions Act, 1926.

Let's examine each of the options:

1. Constitution of a separate fund for political purposes: According to the Trade Unions Act, 1926, a registered trade union is permitted to establish a separate fund from its general fund for promoting the civic and political interests of its members. Therefore, this is a right provided under the Act.
2. Giving membership to a person aged 17 years: The Trade Unions Act, 1926, allows for the membership of individuals who are actually engaged or employed in an industry, whether they are 15 or more years old, so a 17-year-old can indeed be a member. Thus, this is also a right provided by the Act.
3. Committing a tortious act in contemplation of a trade dispute, by an agent of the Trade Union, without the knowledge of the executive of Trade Union: The Act provides immunity to trade unions from certain liabilities in tortious acts committed in contemplation or furtherance of a trade dispute. Hence, this is covered under the Act.
4. Making changes in the employment contract through an act done in contemplation of a trade dispute: This is not a right provided by the Trade Unions Act, 1926. Trade unions cannot unilaterally alter the terms of an employment contract; any such changes would generally require mutual agreement between the employer and the employees under existing legal frameworks rather than through activities specifically sanctioned under the Trade Unions Act.

Based on this analysis, the option that is not a right under the Trade Unions Act, 1926, is:

Making changes in the employment contract through an act done in contemplation of a trade dispute