This is what people are talking about when they say that there's too many cooks in the kitchen!
Volume 3 of SSSS is now open for preorders on the Hivemill store (LINK TO PREORDER HERE) by the publisher Hiveworks! If you also want the signed bookplate or the sticker set that were in the Kickstarter, just wait until after the preorder ends to buy it. (They're still processing the tail end of the Kickstarter orders, which include stickers etc, and they don't want to mess up the inventory count for the warehouse.) You don't need to worry about books running out, they printed enough to last at least a year of regular sales + the Kickstarter for book 4 (which is tentatively planned for the end of this year/early parts of next year.)
(Also, if you are in the EU, if you'd prefer you can buy a copy directly from me here (only book 3, not 1 or 2) and not need to pay customs since I ship from Finland, while Hivemill ships from America.