I have three Flow 2 hives consisting of 2 broods/super each. I don’t have bees yet.
It has been suggested I have yet another, standalone brood box to be prepared for unexpected events. I’m going to do this but need to ask the question “What components comprise a single standalone brood box?”
You’ve got three hives (colonies) in the making if I read your post correctly. Why would you need another one just incase?? Borrow frames from one of the other hives if something goes awry.
If you’re that concerned just get a 4 or 5 frame nuc box and always ensure you have two standard frames of capped honey available in the freezer/wrapped and sealed. Personally, I think it’s overkill though.
The other thing you can do if you want to stick with the Flow Hive 2 design, is purchase a Flow Hive 2, and have the body as a backup in case you want to split your hive in Spring. You would use the wooden components except for the Super and then add this later when the bees have expanded.