1 release a month would be perfect, including a variant option, as then it enables more people to buy if they want. 3 different releases was a bit much but as a 1 of it wasnt to bad but 3 every month would be to much.
Once a month would be the best option for me, mainly because the releases will be spread out and it keeps the excitement going every month. Not to mention, the collection will be more rare compare to releasing multiple at a time (gives more people chances to get their hands on every releases).
I'd prefer just 1 a month, but 2 is still doable. Normally 3 released on the same day would be too much to handle, but in this isolated case we at least knew long beforehand, enough time to secure the funds.