Wedding Announcement keeps its invitation counters as it transforms into Wedding Festivity.

Wedding Announcement's ability looks back in time to the combat phase of this turn, even if it wasn't yet on the battlefield when you attacked. If you attacked with at least two creatures this turn, you will draw a card as the ability resolves. This is true even if one or more of those permanents is no longer a creature or no longer on the battlefield.

Transforming is part of the triggered ability that puts invitation counters on Wedding Announcement. If some other effect causes Wedding Announcement to have three or more invitation counters on it, it won't transform until the next time its triggered ability resolves.