Consider the conversion format 12, which prints the date as yymmdd. The first four characters of that date are what you desire. I'm not sure what you're looking for in terms of 30/60/90 days, but I think this code will help with formatting the date.

DECLARE @today DATE = GETDATE()

DECLARE @tablename30 char(9) = 'table' + SUBSTRING(CONVERT(char(6), DATEADD(mm, -1, @today), 12), 1, 4)

DECLARE @tablename60 char(9) = 'table' + SUBSTRING(CONVERT(char(6), DATEADD(mm, -2, @today), 12), 1, 4)

DECLARE @tablename90 char(9) = 'table' + SUBSTRING(CONVERT(char(6), DATEADD(mm, -3, @today), 12), 1, 4)

SELECT @tablename30 [30 Days], @tablename60 [60 Days], @tablename90 [90 Days]