How do you want your final output to look? Are the feed items from all three feeds going to be in one ordered list where the last will be #15?
Or will these be separate lists?
For the former, it might be easier to just bundle each RSS into it’s own array, then at the end merge the arrays into one, iterate it, then you can use the built-in bundle order position __IMTINDEX__ variable as the first parameter in the switch() function.
Here’s how that would look:
In the Iterator module you’d use the merge() function on the arrays produced by the Array aggregators.
For separate lists, @alex.newpath’s suggestion might be the way to go.
Again, a lot of this depends on what your final output should look like.