How to split this partition without consuming another half a terabyte?

I have a table, RANGE partitioned, with 3 partitions. P3's upper bound is MAXVALUE. Each partition has its own tablespace.  The table has LOBs.

P1 contains 18 gigs, P2 8 gigs and P3 nearly 400 gigs.

I would like to "level" these partitions.  I succeeded in merging P1 and P2 and renaming the resulting partition as P1.  I would now like to split P3 into P2 and P3, but I see from the documentation, I can not "reuse" P3.

I'm new to partitioning, so when I made my first attempt with this, I ended up with what I expected in P2, but the process was not done at that point and I could see that the tablespace holding the original 400 gigs was growing -- not what I wanted.  I don't have a spare 400 gigs (well, I don't want to ASK for an additional 400 gigs) to be able to re-create P3.