id     shipweight1     shipweight2     shipweight3

@id int,
@weight int
as 

if not exists(select id from weights where id = @id)  
   begin
   insert into weights (id,shipweight1)
   values(@id,@weight) 
   end
else if *****

else 
UPDATE  weights
SET     shipweight2 = CASE WHEN shipweight2 is null THEN @weight ELSE shipweight2 END,
        shipweight3  = CASE WHEN shipweight2 is not null and shipweight3 is null  THEN @weight else shipweight3

UPDATE dbo.weights
SET
    shipweight2 = CASE WHEN shipweight2 IS NULL THEN @weight ELSE shipweight2 END,
    --CASE is just to doublecheck -- should always apply because of the WHERE conditions
    shipweight3 = CASE WHEN shipweight2 IS NOT NULL AND shipweight3 IS NULL THEN @weight ELSE shipweight3 END 
WHERE
    id = @id AND
    (shipweight2 IS NULL OR shipweight3 IS NULL)

 DECLARE
	@id int,
	@weight int,
	@Field varchar(12),
	@Result int

 -- Test values
 --SET @iD = 3
 --SET @weight = 28

 IF NOT EXISTS(SELECT id FROM [dbo].[weights] WHERE iD = @id)  
   BEGIN
	INSERT INTO [dbo].[weights](id,shipweight1)
	VALUES(@id,@weight) 
   END
 ELSE BEGIN
    SELECT @Result = shipweight2 FROM [dbo].[weights] WHERE iD = @iD

   SELECT @Field = 
   CASE 
      WHEN @Result <> 0 THEN 'shipweight3'
      ELSE 'shipweight2'
   END

 IF @Field = 'shipweight2' BEGIN
    UPDATE [dbo].[weights] 
    SET   shipweight2 = @weight
    WHERE iD = @iD
 END
 ELSE BEGIN
    -- check first if is null, for not overwrite
    SELECT @Result = shipweight3 FROM [dbo].[weights] WHERE iD = @iD

    IF @Result is null BEGIN
       UPDATE [dbo].[weights] 
          SET	shipweight3 = @weight
       WHERE iD = @iD
    END
 END

END

-- SELECT * FROM [dbo].[weights] WHERE iD = @iD

CREATE TABLE #Weights
    (id INT, shipweight1 DECIMAL(5,2)
    ,shipweight2 DECIMAL(5,2), shipweight3 DECIMAL(5,2))

INSERT INTO #Weights (id, shipweight1, shipweight2)
SELECT 1, 1.1, NULL
UNION ALL SELECT 2, 3.1, 2.3

SELECT * FROM #Weights

;WITH SampleData (id, [weight]) AS (
              SELECT 1, 4.4
    UNION ALL SELECT 2, 5.5
    UNION ALL SELECT 3, 6.6)
MERGE #Weights t
USING SampleData s
ON t.id = s.id
WHEN MATCHED THEN 
    UPDATE SET shipweight3 = CASE WHEN shipweight2 IS NULL THEN NULL ELSE weight END
        ,shipweight2 = CASE WHEN shipweight2 IS NULL THEN [weight] ELSE shipweight2 END
WHEN NOT MATCHED THEN
    INSERT (id, shipweight1)
    VALUES (s.id, weight);
    
SELECT * FROM #Weights

DROP TABLE #Weights