It's a 1-month preorder window. Go back to DCBS and pull up the Marvel/DC/whatever preorders, and any book scheduled for March 2021 release will now be available for you to order. Again, sometimes publishers make books available for preorder much further in advance; DCBS doesn't make that call, the publisher does. For instance, I preordered
Absolute Swamp Thing by Alan Moore Vol. 1 as soon as it was available. Did it show up with all of the other books on that months order? No, because it was solicited further in advance. It showed up a month or two (or three?) later due to DC's delays. But I got it for $50 instead of $100.
If you're asking if you can preorder a book scheduled for March, two for April, and another for May, and get them all shipped to you in June in one big package, the answer is no unfortunately. DCBS follows a strict schedule based on Previews and what's available in their catalog on a given month. But what savings rack up so quickly, the few extra you may spend on shipping is mitigated. And, more importantly, you get the book you want without having to lose sleep the night before hoping that IST or CGN doesn't sell out in 5 minutes.