Try this..

=IF(ISERROR(AVERAGE(IF(ISERROR(SP!C3:E3),FALSE,IF(SP!C3:E3="",FALSE,SP!C3:E3)))),"",AVERAGE(IF(ISERROR(SP!C3:E3),FALSE,IF(SP!C3:E3="",FALSE,SP!C3:E3))))
or edited version of Ron's formula

=IF(ISERROR(AVERAGE(IF(ISNUMBER(SP!C3:E3),SP!C3:E3))),"",AVERAGE(IF(ISNUMBER(SP!C3:E3),SP!C3:E3)))