SELECT
    id,
    a,
    b
FROM prefix_2014.mytable
UNION ALL
SELECT
    id,
    a,
    b
FROM prefix_2013.mytable
UNION ALL
    id,
    a,
    b
FROM prefix_2012.mytable

WITH CTE AS(SELECT
    id,
    3 as [Year]
FROM prefix_2014.mytable
UNION ALL
SELECT
    id,
    2 as [Year]
FROM prefix_2013.mytable
UNION ALL
Select
    id,
    1 as [Year]
FROM prefix_2012.mytable)
Select  ID,MAX([Year]) as YR into #T From CTE
group by ID


Select t.ID,a,b From #T t
join Test1 t1
on t1.id = t.id
where YR = 1
UNION ALL
Select t.ID,a,b From #T t
join Test2 t2
on t2.id = t.id
where YR = 2
UNION ALL
Select t.ID,a,b From #T t
join Test3 t3
on t3.id = t.id
where YR = 3

CREATE VIEW YearIDs AS
SELECT ID, Max(year) FROM
(SELECT
    id, 2014 as year
FROM prefix_2014.mytable
UNION
SELECT
    id, 2013
FROM prefix_2013.mytable
UNION
    id, 2012
FROM prefix_2012.mytable)
GROUP By ID )

SELECT T1.id, T1.a, T1.b
FROM prefix_2014.mytable AS T1
INNER JOIN YearIDs AS Y1 ON Y1.Id = T1.ID AND Y1.year = 2014
UNION ALL
SELECT T2.id, T2.a, T2.b
FROM prefix_2013.mytable AS T2
INNER JOIN YearIDs AS Y2 ON Y2.Id = T2.ID AND Y2.year = 2013
UNION ALL
SELECT T3.id, T3.a, T3.b
FROM prefix_2012.mytable  Y3.Id = T3.ID AND AS T3
INNER JOIN YearIDs AS Y3 ON Y3.year = 2012