The short version
The Hypervolt 3 Pro is a straight generational upgrade over the Hypervolt 2 Pro: more stall force, longer battery, quieter, bigger attachments — and a lower launch price.
When the Hypervolt 3 Pro is the right pick
Buying new, almost always. It’s better on every hardware axis and costs less at list.
When the Hypervolt 2 Pro is the right pick
Only when it’s discounted well below the 3 Pro and you don’t need the extra power or battery.
Also worth comparing
Cross-shop the 3 Pro against Theragun PRO Plus. Full field: best massage guns.