Get Last Name
With Middle Name
=RIGHT(A1,LEN(A1)-SEARCH(" ",A1,SEARCH(" ",A1)+1))
Without Middle Name
=RIGHT(A1,LEN(A1)-SEARCH(" ",A1))
This way will work with both, with or without middle names (maximum of 3 names in total)
=IF(LEN(A1)-LEN(SUBSTITUTE(A1," ",""))=1,RIGHT(A1,LEN(A1)-SEARCH(" ",A1)),RIGHT(A1,LEN(A1)-SEARCH(" ",A1,SEARCH(" ",A1)+1)))