select user, month, month_2, value_3
from (select t.*,
             lead(month, 2) over (partition by user order by month) as month_2,
             (sum(value) +
              lead(value) over (partition by user order by month) +
              lead(value, 2) over (partition by user order by month)
             ) as value_3
      from t
     ) t
where month_2 = add_months(month, 2);