Fill in the gap:
3, 8, 6, 14, \text{________}, 20

This is an alternating number series containing two distinct sub-sequences:
1. First sequence (odd-positioned terms): 3, 6, \text{________}
Pattern: $+3⟹3+3=6$, then $6+3=9$.
2. Second sequence (even-positioned terms): $8,14,20$
Pattern: $+6⟹8+6=14$, then $14+6=20$.

The missing term belongs to the first sequence, which gives $6+3=9$.