An example
create function add2numbers(@a int, @b int) returns int as
begin
declare @returnvalue int
select @returnvalue = @a + @b
return @returnvalue
end
GO
Now in your other function you can call the above.
create function add3numbers(@a int, @b int, @c int) returns int as
begin
declare @returnvalue int
select @returnvalue = dbo.add2numbers(@a, @b)
select @returnvalue = dbo.add2numbers(@returnvalue, @c)
return @returnvalue
end
GO