To get values from other tables or columns of the same table but different row, you should use a function:

CREATE TABLE [dbo].[a2]

(

[a1ID] [int] NULL,

[col3] AS (a1ID^2)

)

GO

create function get_a2_col3(@a1id int)

returns int

as

begin

return (select col3 from [dbo].[a2] where a1ID = @a1id)

end

go

CREATE TABLE [dbo].[a3]

(

[a3ID] [int] NULL,

[col4] AS dbo.get_a2_col3(a3ID) --(dbo.a2.col3 - 1)

)

GO

drop table [a2]

drop table [a3]