Wildcards don't work in direct string comparisons... they instead are used in Searches and Lookups. Try this instead:
IF(OR(Not(Iserror(Search("PBX",E20))),Not(Iserror(Search("SB",F20))),Not(Iserror(Search("ZE",F20)))),"N","Y")
Edit: Thanx Greg

copy and paste strikes again... but I also always forget that Isnumber() will return realiable results in spite of errors, so incorporating NBVC's solution, this would be better written as:
IF(OR(Isnumber(Search("PBX",E20)),Isnumber(Search("SB",F20)),Isnumber(Search("ZE",F20))),"N","Y")
I never can get the {} groupings to work in a way I expect, so I tend to shy away from them... personal preference.