Some of the 3-letter-words starting with J are quite common: JAB, JAM, JAR, JAW, JAY, JET, JIG, JOB, JOG, JOT, JOY, JUG, and JUT. But others are more unusual.
JAG - to cut unevenly
JEE (or GEE) - to turn right. Or the letter G.
JEU - a game (pl is JEUX, worth at least 40 pts when played parallel to POTTING, for example)
JEW (offensive) - to swindle
JIB - to refuse to proceed further
JIN - a supernatural being in Muslim mythology
JOE - a fellow. or coffee.
JOW - to collect a toll
JUN - a coin of North Korea (pl in JUN)
JUS - a legal right (pl is JURA)
All these take an -S, except where a different plural is listed. Note that *JOS is not valid, since the plural of JO is JOES.
There are no 3-letter-words with J in the middle, but there are three that end in J. (All take -ES, but not -S.)
HAJ (or HADJ) - a pilgrimage to Mecca
RAJ - sovereignty
TAJ - a conical cap worn in Muslim countries