Demo, Sql Server - rextester

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CREATE TABLE #Table1
    ([Year] int, [Period] int, [Country] varchar(50), [Value] int)
;
    
INSERT INTO #Table1
    ([Year], [Period], [Country], [Value])
VALUES
    ('2016', '2', 'Morovia', '100'),
    ('2016', '9', 'Morovia', '100'),
    ('2016', '10', 'Elbonia', '-20'),
    ('2016', '10', 'Elbonia', '2000'),
    ('2016', '10', 'Elbonia', '200'),
    ('2016', '10', 'Elbonia', '-100'),
    ('2016', '10', 'Elbonia', '1000'),
    ('2016', '10', 'Morovia', '200'),
    ('2016', '10', 'Elbonia', '-200'),
    ('2016', '10', 'Elbonia', '-200'),
    ('2016', '10', 'Elbonia', '100'),
    ('2016', '10', 'Elbonia', '60'),
    ('2016', '10', 'Elbonia', '40'),
    ('2016', '11', 'Morovia', '200'),
    ('2016', '11', 'Elbonia', '100')

;WITH cte
		 AS (SELECT year,
					period,
					country,
					Sum(value) AS sumvalue
			 FROM   #Table1 t
			 GROUP  BY year,
					   period,
					   country)
	SELECT a.Year,
		   a.Period,
		   a.Country,
		   a.sumvalue + Isnull(Sum(b.sumvalue), 0)
	FROM   cte a
		   LEFT JOIN cte b
				  ON a.Country = b.Country
					 AND Datefromparts(b.[Year], b.Period, 1) IN ( Dateadd(mm, -1, Datefromparts(a.[Year], a.Period, 1)), Dateadd(mm, -2, Datefromparts(a.[Year], a.Period, 1)) )
	GROUP  BY a.Year,
			  a.Period,
			  a.Country,
			  a.sumvalue

View schema
Execution time: 0,03 sec, rows selected: 6, rows affected: 15, absolute service time: 0,17 sec