Step 1: Analyzing group order.
Finite groups with exactly 3 conjugacy classes must have order \( p^2 \) for some prime \( p \).
Step 2: Counting non-isomorphic groups.
For order \( p^2 \), there are exactly two non-isomorphic groups:
1. The cyclic group \( {Z}_{p^2} \).
2. The direct product \( {Z}_p \times {Z}_p \).
Step 3: Conclusion.
The number of non-isomorphic groups is \( {2} \).