1. The same three letters, in the same order, can fill the blanks to complete the words below.

_ _ _ P
_ _ _ M
_ _ _ W
_ _ _ D

2. The same four-letter word will complete the partial words below.

D _ _ _ _
ST _ _ _ _
TER _ _ _ _

Answers:

1. The letters are PRO:

PROP/PROM/PROW/PROD

2. The word is RAIN: 

DRAIN
STRAIN
TERRAIN