This puzzle has two versions:

Version 1:
The same four letters, in the same order, can complete each partial word below. What are the letters, and what would be a good title for this puzzle?

ST
FL
CH
ECL

Answers:

The letters are AIRS:

STAIRS
FLAIRS
CHAIRS
ECLAIRS

A clever title for this puzzle would be PUTTING ON AIRS.

Version 2:

The same three letters, in the same order, can complete each partial word below. What are the letters, and what would be a good title for this puzzle?

STA
FLA
CHA
ECLA

Answers:

The letters are IRS:

STAIRS
FLAIRS
CHAIRS
ECLAIRS

A clever title for this version of the puzzle would be TAX EVASION.