Dear Anonymous A,
Sid is right on this.
More so, this disagreement likely stems from what might have been perceived as weak points, not areas where you performed well. In other words, they're likely not asking you to come back to demonstrate areas where they believe you're amazing; rather, to prove to them that in area in which you were variably perceived to be weak is not a fatal one that is likely to repeat.
So, what can you do? 3 things, I believe:
1. Conduct a brutal self-assessment from your first two interviews to try to deduce what this problem area might be: are your quantitative skills wobbly? Do you have trouble synthesising? Was your structure unsound? Are you unimaginative with brainstorming? Are your processes clear, logical, and replicable? Do you bring sound judgement to your decision-making? et. cetera
2. Develop an effective plan to overcome this problem area and work to weave it into any Case prep you do between now and then. Although, please don't go crazy on the Case prep: the fact that they've asked you to come back shows they have at least some confidence in your Case skills more generally.
3. Go back to your third interview with confidence and calm and with the reassurance that you have given your best in 1) and 2) above.