Hi Shaimaa, they all mean being away from someone or something
"Absent" is an adjective.
Ex: The absent boy missed a great party. ("Absent" is describing "boy.")
Ex: She was sad because the boy she likes was absent from class. ("Absent" is describing "boy.")
"Absence" is a noun.
Ex: Your absence is inexcusable. ("Absence" is serving as the subject of this sentence.)
Absenteeism is also a noun
Ex: The boss told me, " I don't appreciate your frequent absenteeism. One more day away and you're fired!" ("Absenteeism" is what you have done.)