Right now it's Bamboo turn to be in the ancient flower knight re-gacha. Corbett will be in the next (cost 5oo FS though, not 3oo). She will be there before spring I guess. Adlay and Zinnia may never came again. Or at least, not before a little while.
Pavonia is only available in the Rainbow Medal shop for 300 of them. Here again, just wait and see. And pray to see her comming soon. But I think she will show in the second half of the year. Maybe even after summer.
3oo FS gacha are not really rare or common. They are mainly available for special holidays like Chirstmas, Summer (with Swimsuits), etc. The guaranteed next one will be for Valentine event with Valentine girls. Then, we should have Easter on... easter, Brides in June, several Swimsuit across Summer, Halloween, several popularity contests winners, Christmas and New Year again.
Random 3oo gacha may appears out of nowhere like the recent Cattleya house one but thoses are much rare and imprevisibles.
But things may change at anytime depending on devs and marketing team will.