create table #vendor (
   VendorName varchar(20)
   , Address1 varchar(20)
   , Address2 varchar(20)
)

insert #vendor values ('A', 'A', 'c/o')
insert #vendor values ('B', null, 'c/o')
insert #vendor values ('C', 'B', null)
insert #vendor values ('D', null, 'c/o')
insert #vendor values ('E', null, 'c/o')

select * ,
Rules = 
Substring(Cast(
   Case When VendorName = Address1 then ',1' else '' end + 
   Case When Address1 = 'c/o' or Address2 = 'c/o' then ',2' else '' end + 
   Case When VendorName is not null and Address1 is null then ',3' else '' end 
as Varchar(31)), 2, 30)
from #vendor

drop table #vendor

-- here is the output
VendorName      Address1        Address2        Rules                
--------------- --------------- --------------- -------------------- 
A               A               c/o             1,2
B               NULL            c/o             2,3
C               B               NULL            
D               NULL            c/o             2,3
E               NULL            c/o             2,3