How to calculate quantity from different table ?

Not sure with your question,Hope this may help you.

If your column datatype are similar you can use 'union all' in sql.
If you need list of Quantity column,then try like this,

declare @ItemId int=1;
with cte as (select Quantity,ItemId from tbl1 a union all 
select Quantity,ItemId  from tbl2 b union all 
select Quantity,ItemId  from tbl3 c) select Quantity from cte where ItemId=@ItemId

//It will return list of quantity column

If you need Sum of quantity,Do like this,

declare @ItemId int=1;
with cte as (select Quantity,ItemId from tbl1 a union all 
select Quantity,ItemId  from tbl2 b union all 
select Quantity,ItemId  from tbl3 c) select SUM(Quantity) as TotalQuantity from cte where ItemId=@ItemId