Do we know that there are always going to be 3 characters?
I would look at the INSTR function.
myString="sdf abc eee abd xqq"
? instr(mystring,"ab")
5
mystring=mid(mystring,1,instr(mystring,"ab")+1)+mid(mystring,instr(mystring,"ab")+3)
? myString
sdf ab eee abd xqq
of course you would have to set up a loop and check for a second occurance of 'abX'
Simi