select t.id, t.a, t.b, t.c 
  from (select id, a, b, c
	  from test_table
	 where (a = $1 or $1 is null)
	   and (b = $2 or $2 is null)
	   and (c = $3 or $3 is null)) t 
 where t.a = 1
union 
select t.id, t.a, t.b, t.c 
  from (select id, a, b, c
	  from test_table
	 where (a = $1 or $1 is null)
	   and (b = $2 or $2 is null)
	   and (c = $3 or $3 is null)) t 
 where t.a = 2
union 
select t.id, t.a, t.b, t.c 
  from (select id, a, b, c
	  from test_table
	 where (a = $1 or $1 is null)
	   and (b = $2 or $2 is null)
	   and (c = $3 or $3 is null)) t 
 where t.a = 3

select t.id, t.a, t.b, t.c 
  from (select id, a, b, c
	  from test_table
	 where (a = $1 or $1 is null)
	   and (b = $2 or $2 is null)
	   and (c = $3 or $3 is null)) t 
 where t.a in (1,2,3)

select id, a, b, c
  from test_table
 where a in (1,2,3)
   and (b = $1 or $1 is null)
   and (c = $2 or $2 is null)

select id, a, b, c
  from test_table
 where a in (1,2,3)
   and (a = $1 or $1 is null)
   and (b = $2 or $2 is null)
   and (c = $3 or $3 is null)