After Mist has gone missing in the Snow Ruins BF3 you will (of course) need to get her back. To do that you will need to get the 'Grimoire of Time' (a book) of which you should have gotten right after mist went missing. You will not be able to read it and will need to find someone to read it. Give the book to Kanno to help you read it. After around four days go talk to him again. He will continue to tell you a story that explains the book. After that he gives you twelve 'Crystal Seeds'. You will need to give the seeds to all the marrage canadates besides Mist. Give the seeds to the following girls. Lara, Anette, Eunice, Melody, Iris Noire, Iris Blanch, Rosetta, Selphy, Cinnamon, Uzuki, Bianca, and Tabatha.
After giving all the Crystal Seeds to the girls, you will have to wait three to four days. It does not matter the order that you get the flowers back in, just that you get them back. You will get different flowers from different girls. Uzuki-Aquamarine Flower, Lara-Lapis Lazuli Flower, Selphy-Saphire Flower, Bianca-Peridot Flower, Iris Noire-Pearl Flower, Iris Blanch-Pearl Flower, Anette-Topaz Flower, Tabatha-Emerald Flower, Rosetta-Opal Flower, Melody-Amethyst Flower, Eunice-Garnet Flower, and Cinnamon-Ruby Flower. Go talk to Kanno after you've gotten them all and he'll tell you to put them on the pedistal. Go inside the room to his left and up the stairs to the pedistal. Put all the flowers into it and Kanno will come in. He will tell you that you have only eleven in and to put the last one in. You tell him there all in and he wonders. He'll say outloud that unless the same girl grew two seeds they all should have been different. You two will puzzle for a minute before you realize the two Iris's are the same girl, and therefore have the same personality. You ask Kanno to give you another seed and figure you will give it to Candy. He gives you bad news as there are no more seeds left. You run out shouting you WILL find another seed, you WILL rescue Mist.
Finding the Last flower
The last flower is located in front of Mist's House: in the pot she used to water in the morning.