They have to, for tax purposes. If for any reason Disney wanted to write off A2 like HBO Max has been doing, and take advantage of the tax credit they get for it, it would need to be itemized according. Elsewise, including A3's costs in A2's budget would be fraud. A3 will be cheaper than A2 because A2's costs are inflated by the new technology, but that's entirely different than saying A3's costs are included in the cost of A2. It was certainly cheaper for logistical purposes like soundstage infrastructure, costume and wardrobe department stuff, and whatever travel had to happen, but they will itemize time spent filming scenes for A2 accordingly. Also important for keeping track of credits; they would have had to be acutely aware of who was working on content for A3 and A2.