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

TU _ _ _
CA _ _ _ H
EC _ _ _ SE

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

_ _ _ DE
_ _ _ SE
_ _ _ LE

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

_ _ _ ON
_ _ _ SS
_ _ _ UP
_ _ _ WD
_ _ _ WN

Answers:

1. The missing letters are LIP:

TULIP
CALIPH
ECLIPSE

 2. Each word can be preceded by GUI:

GUIDE
GUISE
GUILE

2. Each word can be preceded by CRO:

CROON
CROSS
CROUP
CROWD
CROWN