I know for a fact that a couple of the girls at BSS used to do threesomes. I have had one with Vivian and Jennifer, admittedly that was a few years ago and I have not been in since before COVID. I tend to stick with massages now as I am getting on a bit and a lazy git but good massages are few and far between in Edinburgh at the moment. I would phone your chosen parlour and ask or go in for a one to one and ask.
Good luck.