This is going to sound crazy, and I may be completely off-base,
I but I feel like the order of when the calculation was created
determined the order in the tooltip.
What I did was create three sets of identical calculations
of Furniture, Office Supplies, and Technology,
but I created them in three different orders.
(i.e. I created Tech first in the A set, second in the B set
and third in the C set). Then I added the Measures
in any order onto the different sheets.
In the composite screenshot below, you can see that order
of presentation in the tooltip is always 1-2-3,
and I think that holds regardless of how the Measures get
added to the marks card (second screenshot below).