Crystal Reports Parameter

Message

   I create 3 parameters...but how can i create the formula, if i want to choose 1 or 2 parameters and the others are blank.
Right now i'm using 'and' statement...how can i used 'or' statement for this parameter...

{NPCS_INBND_FILE_HDR.VOLSER_NO} = {?Volser} and
{STATUS.STATUS_DESC} = {?File Status} and
{NPCS_INBND_FILE_HDR.NPCS_CYCLE_NUM} = {?Initial Cycle ID}

let say, i want to choose Initial Cycle ID is blank and the format is number...please help me to create the formula...

Thank You.