As I tested, the syntax of the measure is right, and get the expected result. This measure will calculate the Total services since the last 90 days. But for your requirement, the TODAY() function is inappropriate. I try to reproduce your scenario, and get expected result as followings.
The first screenshot is the sample data.
Then I create a measure to calculate the rolling 90 day average and get the result shown in second snapshot.
This makes sense. However, I don't need the ability to look at rolling totals on historic dates, only in real time. So if I look at the Dashboard this morning with refreshed data, I would see a rolling total 90 days before today, and then when I check tomorrow it will be 90 days from tomorrow.
I'm really bad at using "date" tables and was trying to avoid that if possible!