I want to know about Fish and Flower Garden, is it possible and worth it when winter comes?

Yes, it's still worth to tour around the lake in winter. Notably, the winter scenery is also famous and impressive. In addition, you need also visit those two scenic spots you mentioned.
You can use one day to fully appreciated the beatutiful scenery of the lake. After that, half a day is enough to visit the other two. Have a good trip!