Without more information, I would say you could do it like this.

CREATE function [dbo].[myfunc]

( @a int, @b-2 int, @C money)

returns table

AS

return

select d = (isnull(sum(d.cola),0)+@c) * -1

from (Select distinct c.cola

from tab1 b

join tab2 c

on b.cola = c.colb

where b.cola <= @b-2 and

b.colb = @a) c

join tab3 d

on c.cola = d.colb and

@a <> d.cola