The same three letters, in the same order, will compete the words below. What are the letters?

1. CA _ _ _ H 

2. CA _ _ _ ER

3. EC _ _ _ SE

4. EL _ _ _ SE

 

HINT:
The three letters spell a common word.

 

Answer:

 

The letters are LIP:

 

1. CALIPH

2. CALIPER

3. ECLIPSE

4. ELLIPSE