I think the missing letter sequence could be:
LAN
Using the following words:
LANDING - a level of a building between flights of stairs;
BLANKET - a bedspread;
BALANCE - center of balance;
IRELAND (or ICELAND, ZEALAND or many others) - a place;
CATALAN - a language most commonly used in parts of Spain.
I had also considered:
the three-letter sequence ALI, since ALIGNED (level), REALIGN (center), SOMALIA (place) and BENGALI (language) all fit, but I could not for the life of me find a satisfying _ALI___ for 'spread'. Then again, it would not surprise me if the OP's intended answer is neither of these but something else altogether!