1. The same three letters, in the same order, will complete the partial words below:

AP _ _ _ D
DE _ _ _ D
IM _ _ _ D

2. The same four letters, in the same order, will complete each partial word below:

R _ _ _ _
F _ _ _ _
K _ _ _ _
SK _ _ _ _
WR _ _ _ _
STR _ _ _ _ 

Answers:

1. The letters are PEN.

APPEND
DEPEND
IMPEND 

2. The letters are ETCH.

RETCH
FETCH
KETCH
SKETCH
WRETCH
STRETCH