1. The partial words below can be completed by inserting the same three letters, in the same order.

DE _ _ _ S
FA _ _ _ C
HY _ _ _ D

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

S _ _ _
VA _ _ _
PAL _ _ _

HINT:
2. SQUA _ _ _ could be added to the list.

Answers:

1. The letters are BRI:

DEBRIS
FABRIC
HYBRID

2. The letters are LID:

SLID
VALID 
PALLID
(SQUALID)