I posted the round on the 29th and it runs until the 16th so that's the correct time span, but I didn't want to tag it because it's a regular challenge, that I would have posted anyways, not special for the Three Weeks

Anything happening exclusively on DW for the three weeks can absolutely use the tag, even when part of a regular posting schedule :)

Same for anything else you or anybody else posts exclusively to DW during the fest.

could I link back/tag that for Three Weeks if it ends after the three weeks is over?
Any challenge that goes up during the three weeks can use the tag, anything that goes up after the three weeks, I'm thinking no.

Hope that clears it up?

Yes, it does thank you! ^_^