I haven’t seen what the output looks like for preallocated devices in V3, but try:
symcfg list -sid xx -pools -thin -v -detail
Then look at the section “other thin devices with allocations in this pool” It has for each device “Total Tracks”, “pool Allocated tracks” and “pool used tracks”
BTW, for VMAX 3, is there a particular reason you are pre-allocating? Certainly prior to V3, there was a little latency overhead for allocating tracks on first write to the newly allocated space, but much improved in V3, so this should not be a consideration now.