I had a case similar to this where I was able to sync up the x axis on three worksheets. In my case the x-axis was a date field. I happened to know that the data began for one of the worksheets a specific number of months before the other two.
I created a calculated field that determined how many months from the last data point I needed the axis to be. I used that as a secondary axis which only had one point on them. I did dual axes, synced them, and then changed the mark on that axis to a white color so it wouldn't be visible.
I called the calculation 'conditional Axis beginning' and used :
DATEADD('month', -11, [Latest Discharge Date])
Latest Discharge Date is another calculated field that determines that latest date in my data.
Perhaps you could create a field that determines the largest value in your data and then set that on a secondary axis changing the mark to white. I know the cases aren't exactly the same, but maybe the idea will help you.