I think neo has enough booster from the factory. If it's still too soft for you, try the non-neo version. Or you can try 40-41 degrees. If you get non-neo version and it feels too dead, you can add one layer of glue, then one layer of Falco Tempo Long, and then one more layer of glue.
39 degrees is the softest that you can get for blue sponge Chinese rubber, but it's not so soft when compared to ESN/hybrid rubbers. For instance, I think D09c is around 38 degrees on the Chinese scale and D05 is around 36 degrees. So 39 degrees is still pretty hard. But I would agree with Doralikesmath: don't boost it further.
If you haven't tried H3 before, then 39 degrees neo is a good introduction which will let you see if you like hard Chinese rubbers. The looping power you can get from a hard rubber is extremely satisfying. Maybe you will catch the hardness virus like me and end up playing 42 degrees unboosted