What should come in place of the question mark ($?$) in the following series?
$$3,8,6,14,?,20$$

The given sequence is an alternating series comprising two independent nested patterns:
1. First series (odd positions): $3,6,?$
The pattern involves a progressive addition of $3$:

$$3+3=6$$

$$6+3=9$$

2. Second series (even positions): $8,14,20$
The pattern involves a progressive addition of $6$:

$$8+6=14$$

$$14+6=20$$

Following the first alternating rule, the missing term is $9$.