1. What do the following words have in common?

TEST
FUSE
DUCT

2. What do the following words have in common?

END
ION
ALLY

3. What do the following words have in common?

END
LIFE
BROW

HINTS:
1. Partial words FESS and GRESS could be added to the list.
2. The same three letters will change each word into a new one.

Answers:

1. Each word can be preceded by both  PRO and CON.

2. Each word can be preceded by LEG:

LEGEND/LEGION/LEGALLY

3. Each word can be preceded by both HIGH and LOW.