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My best friend has wanted to go on vacation with me for a long time, and we finally planned to go this summer together. My friend will already be in that country for a few weeks due to other reasons, but I will join later on to travel together and eventually fly home together. At the same time my SO asked me if we could go on a holiday together, but I told them I already promised my best friend to go together but we could plan another holiday to a cheaper and closer destination. My SO got a bit upset and sad, since they would want to go to the same location as my best friend and I had planned. My SO did accept it, but I could tell that going together on my vacation would be something they wanted.

After this I talked to my best friend in just enquiring, not pushing at all, if there is a possibility for my SO to tag along. I emphasized if my best friend is not comfortable with my SO tagging along, i would not even invite my SO but in the talk it became clear it would not be a big problem for my friend. Sure it would be nice to travel together just the two of us, but understanding I have relationship, my friend has been kind enough to say if I invite my SO, my friend would just invite their own SO so it would not be a possible awkward trio holiday. My SO and best friend have known each other since before we became a couple, and get along quite well.

After confirming this again with my best friend, if they honestly would not mind me inviting another person, in which they reassured me its okay I thus invited my SO to tag along, which they gleefully accepted. However my best friend's SO does not want to go together anymore, which leaves the three of us again. I honestly could not think of a better way to spend my holiday than with my best friend and SO travelling together, but now I am getting worried how my best friend will not have the vacation they wanted and I will unintentionally wil make them feel like a third wheel or vice versa with my SO.

I feel my best friend would be the happiest if my SO would not tag along, since we in that case would be only the two of us which they wanted in the first place, but to be honest I do not want to de-invite my SO. I feel like both my best friend and SO will be worried about some arrangements, like seatings and sleeping arrangements. Am I the asshole for not de inviting my SO and wanting to take both of them? 