In an attempt to keep thing simple, I created this measure:
TestM = SUMX(VALUES(Dates[Date]), [Attrition Rate])
and then using the filter pane, limited it to the last 3 months using a relative date filter. I think it gets the correct answers.
I suggest testing at your side and you could maybe incorporate the 'last 3 months' part in the measure if you don't want to use the filter pane.
Let me know how you get on.