Well to be fair, those dates, doesn`t mean is the time of filming, is just a timeline for the dancers to get a notion of how many time they will be on this, they need to learn the dance moves over and over again.
I don´t see them filming 3 episodes, one of them musical all together, however i see them filming 618/619 or 621/622 together,in fcat i would bet for the last 2 be filming together bc they done that before, most likely those 2 epis will air on same day so they can sort of blend and to avoid spoilers since those 2 epis may be the last epis of the show or set a new direction to possible seasons and if they end filming in beginning of april, then the last 2 weeks of march can be to film those 2 episodes.
"I offended you with my opinion? Ha, you should hear the ones I keep to myself".