Turns out it was a wise move to cut down from 1 large checked suitcase and 2 carry-on rolling suitcases to 1 of each.
We were able to squeeze in the two bags as well as a duffle and some shopping bags, but that pretty much squeezed it to the limit. There is no way we could have fit three suitcases in there (unless maybe they were all of the carry-on variety).
One other thing I learned about convertibles is that the back seat is a total fiction. For all intents and purposes an adult cannot fit back there. It's extra storage space and that's about it. (The only reason I mention this is because a lot of people asked the question when I was googling the trunk space issue, e.g. 4 people going from Europe to Florida and wondering if they could all fit in the Mustang they rented... uhh, nope.)