Identify the pattern in each sequence and find the missing number.

$8 - 3 = 5$
$18 - 8 = 10$
$38 - 18 = 20$

The differences are $5, 10, 20$, which means each difference is double the previous one. The next difference will be $20 \times 2 = 40$.

$38 + 40 = 78$
The completed pattern is $3, 8, 18, 38, \boxed{\textbf{78}}$.

B) The pattern is to add 7 to the previous term.

$14 - 7 = 7$
$21 - 14 = 7$
$28 - 21 = 7$
$35 - 28 = 7$

The missing number is already filled in the image as 21, which is correct.
The completed pattern is $7, 14, \boxed{\textbf{21}}, 28, 35$.

C) The pattern is to subtract a number that increases by 1 each time.

$23 - 25 = -2$
$20 - 23 = -3$
$16 - 20 = -4$

The differences are $-2, -3, -4$. The next difference will be $-5$.

$16 - 5 = 11$
The completed pattern is $25, 23, 20, 16, \boxed{\textbf{11}}$.

D) The pattern is to subtract numbers following the sequence $12, 4, 6, 8, \dots$.

$21 - 33 = -12$
$17 - 21 = -4$
$11 - 17 = -6$

$12, 4, 6$. The pattern for these subtractions is to subtract 8, then add 2, then add 2. So, $12 \xrightarrow{-8} 4 \xrightarrow{+2} 6 \xrightarrow{+2} 8$. The next number to subtract is 8.

$11 - 8 = 3$
The completed pattern is $33, 21, 17, 11, \boxed{\textbf{3}}$.

E) The pattern is to add a number that doubles each time.

$7 - 4 = 3$
$13 - 7 = 6$

The differences are $3, 6$, which means each difference is double the previous one. The next difference will be $6 \times 2 = 12$.

$13 + 12 = 25$

The next difference would be $12 \times 2 = 24$.
$25 + 24 = 49$. This matches the given number.
The completed pattern is $4, 7, 13, \boxed{\textbf{25}}, 49$.

F) The pattern is to divide the previous term by 3.

$243 \div 3 = 81$
$9 \div 3 = 3$

Apply this to find the missing term.
$81 \div 3 = 27$

$27 \div 3 = 9$. This matches the given number.
The completed pattern is $243, 81, \boxed{\textbf{27}}, 9, 3$.