Step 1: Concept
The number of terms in the multinomial expansion of $(x_1 + x_2 + \dots + x_r)^n$ is given by the formula $^{n+r-1}C_{r-1}$.
Step 2: Meaning
Here, we have $3$ variables ($x, y, z$), so $r = 3$, and the power is $n = 10$.
Step 3: Analysis
Substitute $n = 10$ and $r = 3$ into the formula:
\[ \text{Number of terms} = ^{10+3-1}C_{3-1} = ^{12}C_2 \]
Calculate $^{12}C_2$:
\[ ^{12}C_2 = \frac{12 \times 11}{2 \times 1} = 6 \times 11 = 66 \]
Step 4: Conclusion
The expansion of $(x + y + z)^{10}$ contains exactly $66$ distinct terms.
Final Answer: (B)