It's a countdown. What he is actually saying is "Three hundred... thirty...three" and then you hear them firing.

It's the traditional Soviet arty command for fire. A song featuring it has become a hit and the term has begun to be used by pro-invasion Russians to comment on propaganda videos.

The Ukrainians are probably using it as a form of ironic comedy. Because Russians are the ones being fired upon.

Seemed to coordinate with firing.