Concept:
Endogamy refers to a marriage practice where individuals marry within their own social group.
Step 1:
Endogamy means marriage within the same caste, class, tribe, religion, or social category.
Step 2:
It helps preserve social identity, traditions, and group boundaries.
Step 3:
Exogamy refers to marriage outside one's own social group.
Step 4:
• Same sex background is unrelated here.
• Financial position alone does not define endogamy.
• Marriage between different social categories describes exogamy.
Step 5:
Hence, the correct answer is:
\[
\boxed{\text{(3) Marriage between people of same social category}}
\]