1. What do the following words have in common?

RUN

SUNG
WAVER

2. What do the following words have in common?

FLU
APE
MAN
LATE

3. What do the following words have in common?

COAL
CORAL
TRUNK
GERMAN

HINTS:
1. The same letter, in the same place, will change each word into a new one.
2. The same letter, properly placed, will change each word into a new one.
3. With a change in spelling, but not pronunciation, each word can be followed by the same three letters.

Answers:

1. The letter I can be inserted in each clue to make a new word:

RUIN
SUING
WAIVER

2. The letter X can be added to each clue to make a new word:

FLUX
APEX
MANX
LATEX

3. With a change in spelling, the letters ATE can be added to each clue to form a new word,  while retaining the pronunciation of the clue word.

COLLATE
CORRELATE
TRUNCATE
GERMINATE