I think this is a very difficult situation and you have my empathy and understanding, op.
I am in somewhat of a similar situation because my first child was a prem stillborn, so the child I currently carry will always seem to me and DH as our second child, but to the outside world, he or she will always appear to be the first.
The difficulty is that whenever anyone asks you how many children you have, this issue is always going to arise. The question, which is fairly commonplace, is always going to be a trigger for you. My advice would be to decide how you are going to manage it now and for the future.
One of the things to consider here is how much you want to tell other people. I, personally, feel that our experience with our first baby is a very private matter and not for public consumption, so do not want to invite questions by saying our second baby is our second, which then may lead to enquiries about the first.
More pertinent for your circumstances, I also do not want to give myself further upset in additional to the memories and pain if a close family member refers to my second as my first. To this end, I am going to let this sort of thing go -- even if my mother or MIL does it.
Whenever anyone refers your children and "numbers" or birth order, it will always cause pain for you by conjuring up memories of your second child, regardless of whether they refer to your third child as your third or your second. The question you need to answer is the extent to which you wish to provoke further pain and sadness for yourself on top of the pain of those memories by getting upset about their mistaken comments about birth order.
When it comes to the loss of a child, we need to protect ourselves. It is such an enormous pain, and one that will never truly ease; it is a pain that tattoos your soul. And it is a pain so unfathomable, so fundamental, that I think we need to take care not to inflame the wound further by reacting to what other people say.