Get records between time span
Deeptechtons, we really would like to see your answer, but since you haven't posted it yet, here is how it could be done.  This SQL is for Oracle not SQL Server, so it would have to be translated.  There are details missing from your post, so some assumptions used are likely invalid.

with AllEmployees As 
   (select distinct PersonName from samples)
select * from AllEmployees 
minus
select PersonName from samples 
where trunc(not_avl_date) = trunc(sysdate) 
and (not_avl_for = 1
   or (not_avl_for = 2 and :timespan LIKE '%AM%')
   or (not_avl_for = 3 and :timespan LIKE '%PM%')
);


Sample data:

create table samples (rid Number(3), PersonName Varchar2(20), 
   not_avl_date date, not_avl_for integer);

insert into samples values (1,'Peter',sysdate-2, 1);
insert into samples values (2,'Peter',sysdate-1, 2);
insert into samples values (3,'Paul',sysdate-1, 3);
insert into samples values (4,'Sue',sysdate, 3);
insert into samples values (5,'Bob',sysdate-2, 2);
insert into samples values (6,'Zach',sysdate+1, 1);