If you are looking to simulate a computed column across three tables (and it sounds as if you are), consider a view:

USE tempdb;

GO

-- Three related tables

CREATE TABLE dbo.T1 (PK INT IDENTITY PRIMARY KEY, A INT NULL);

CREATE TABLE dbo.T2 (PK INT IDENTITY PRIMARY KEY, B INT NULL);

CREATE TABLE dbo.T3 (PK INT IDENTITY PRIMARY KEY, C INT NULL);

GO

-- Sample data

INSERT dbo.T1 (A) VALUES (1), (2), (3);

INSERT dbo.T2 (B) VALUES (-4), (-5), (-6);

INSERT dbo.T3 (C) VALUES (14), (28), (36);

GO

-- View to create a sort of 'cross-table' computed column

-- Index the view if appropriate, to materialize the 'computed column'

CREATE VIEW dbo.V

WITH SCHEMABINDING

AS

SELECT T1.PK,

Value = T1.A * PI() + T2.B + LOG10(T3.C)

FROM dbo.T1

JOIN dbo.T2 ON T2.PK = T1.PK

JOIN dbo.T3 ON T3.PK = T2.PK;

GO

-- Show the contents of the view

SELECT PK, Value

FROM dbo.V;

GO

-- Tidy up

DROP VIEW dbo.V;

DROP TABLE dbo.T1, dbo.T2, dbo.T3;

Do not be tempted to use a T-SQL UDF to access data. Bad things will happen to you. 😉