I have 3 chairs that came with an old table I purchased and would love to find another like them. I know finding one like them is a longshot, but it's worth a try.
Can't help you, but oh they're pretty.
It looks to be Pennsylvania Dutch, or maybe German. There's a possibility they were handcrafted by an amateur craftsman(not professionally schooled), so therefor the fourth could have been broken and disregarded. It's very unique and I would try to find 2 chairs thar are a similar color wood, and use 2 of the one pictured. The extra third one could be set in a corner and used when an 'extra' is needed.
We have an antique set like this in a vacation home and we had the same problem. We found a few other chairs of the same vintage and mixed and matched. It adds character. I might take two of these and another pair or two different chairs and mix it up. Maybe put the third chair aside as suggested by the previous poster.