If each of your step levels is using a separate paratag, then it can be done using the <$paranum[paratag]> building block. Assuming that you have something like Step, Sub_Step, Sub_subStep as the three levels of paratags, then to access, the previous numbers, you would use a construct like:
See Step <$paranum[Step]> <$paranum[Sub_step] <$paranum>, where the x-ref targets the (3). in the Sub_subStep paratag.
The reason that this works is that FM will search backwards (i.e. towards the start of the flow) to satisfy the first two building blocks. However, if FM doesn't find a match, then it won't output anything for that building block.
