A [ID, column1, column2, column3]
B [ID, column1, column2, column3, column4]

C [ID, column1, column2,column3]

UPDATE C
  set column1=t.firstTab, column2=t.secondTab, column3=t.thirdTab
  from (select A.column1 as firstTab, B.column2 as secTab,
              (A.column1 + B.column2) thirdTab
        from A, B limit 1; ) as t ;

UPDATE 0

select A.column1 as firstTab, B.column2 as secTab, (A.column1 + B.column2) thirdTab
            from A, B limit 1;

UPDATE C
SET    column1 = A.column1
     , column2 = B.column2
     , column3 = A.column1 + B.column2
FROM   A
JOIN   B ON A.id = B.id  -- ??? not specified in question!
WHERE  C.id = A.id      --  ??? not specified in question!
AND   (C.column1, C.column2, C.column3) IS DISTINCT FROM
      (A.column1, B.column2, A.column1 + B.column2);

UPDATE C
  set column1=t.firstTab, column2=t.secondTab, column3=t.thirdTab
  from (select A.column1 as firstTab, B.column2 as secTab,
              (A.column1 + B.column2) thirdTab
        from A
        join B  on ...
        where ... ) as t