Concept:
In series connection of capacitors, charge on each capacitor is same and voltage divides.
Step 1: For capacitors in series:
\[
\frac{1}{C_{\text{eq}}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}+\cdots
\]
Step 2: The equivalent capacitance in series is always less than the smallest capacitance.
Step 3: Therefore, the correct relation is:
\[
\boxed{\frac{1}{C_{\text{eq}}}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}}
\]