You put them all in one if-then statement. You need to have the same number of endif at the end of the string as "if".
if asm_mbr_part_no== "PRT0003"
bom_qty=3.5
else
if asm_mbr_part_no== "PRT0004"
bom_qty=6.2
else
if asm_mbr_part_no== "PRT0005"
bom_qty=1.5
else
bom_qty=rpt_qty
endif
endif
endif