1. What do the following three-letter words have in common?

BUS
RUB
PIT
WAR

2. What do the following words have in common?

ENDS
LEVEL
SECOND
DECISION

3. What do the following words have in common?

RAP
FLU
CUP

Answers:

1. The letter Y can be added to each word to form a new one:

BUSY -RUBY - PITY - WARY

2. Each word can be preceded by SPLIT.

3.  The letters ID can be added to each word to form new words:

RAPID - FLUID - CUPID

Credits: 
#1: LA Times Crossword - Aug. 2, 2009
Nancy Salomon
AT THE Y

#3: LA Times Crossword - Sept. 27, 2009
Alan Arbesfeld
PUT A FINGER ON IT