I saw someone saying flushing the tree. What exactly is flushing? Cutting the fronds or giving nutrients

By the way it has been replanted 3 weeks ago.
The fronds didn't change color just drooping like it has no energy

Maybe just transplant shock? Were the roots disturbed in any way? Where did you get it and where are you located?

Trachys do not transplant well from balled and burlapped specimens or otherwise bare-rooted (shipped w.o. a pot). A few years ago, I ordered a nice big one with this tiny root ball all wrapped up in plastic. Leaves started folding closed from Day 1 even with a good watering (assume this what is meant by 'flooding'). Don't feed it if it is struggling--will only make it worse! Mine did not make it, yours might. If I were to get a similarly shipped palm now, I would probably immediately containerize it and position it in the shade for a few weeks, then, after it recups from the shipping, plant it in the ground.
P.S., never had a problem with shipped CONTAINERIZED palms.

I've experienced this before. Definitely transplant shock, but it's not bad. It's late in the season but hopefully it will be able to establish a better root system before the cold sets in. You should not treat this differently than your other Windmill except maybe some extra winter protection. The existing fronds will remain droopy and sad, but next season the new ones should come in fine. Most likely next season will only yeild a few fronds as the tree will continue to focus on reestablishing its root system. I see you're in zone 7. Check often for any sign of crown rot next April/Maywhen it is most vulnerable. Dust the crown with a bit of Garden Dust to help prevent any mold, fungus or bugs.