Solutions :

3-letter words (1 point): 13

BES; BIS; BIT; EFS; EMS; FES; FIB; FIT; IFS; SIB; SIF; SIT; TIS.

4-letter words (1 point): 4

BISE; FIBS; SIBS; SIFT.

5-letter word (2 points): 1

BESIT.