Assuming time part is not to be considered and that you need this for only 3 date spans (if you need for more one could Recursive CTE for this) :

-- The two dates for first condition

select DATEADD(m,DATEDIFF (m,'19000101',GETDATE()),'19000101')

select GETDATE()

-- The two dates for second condition

select DATEADD(m,-1,DATEADD(m,DATEDIFF (m,'19000101',GETDATE()),'19000101'))

select DATEADD(m,-1,GETDATE())

-- The two dates for third condition

select DATEADD(m,-2,DATEADD(m,DATEDIFF (m,'19000101',GETDATE()),'19000101'))

select DATEADD(m,-2,GETDATE())

The only variable in these 6 dates is getdate() (which of course can be replaced with any other date u want)...