Read this post:
We are all getting third hand indistinct information. I suspect it's possible that they initially said they would plan for a NFZ to last anytime between those three days, and then as things firmed up for their launch times they were able to also give a more specific time...
www.sinodefenceforum.com
And the following 3 posts.
TL: DR It's more likely that China announced that there would be a no fly zone
sometimes during those 3 days due to satellite debris, but it might then have been spurn by some media to be that China wanted to impose 3 days of no fly zone
coughReuterscough
Also, if they actually wanted to impose a3 days no fly zone for some military exercise, we would likely have posters posting that news from first hand sources (from the PLA? Chinese Foreign ministry? I dunno, but actual first hand source)