I think you and your dp need to have an in depth, honest discussion about this potential third dc.
Firstly, you both need to have a deep think about the degree to which you each want a third child - try picturing yourselves, 5-10 years from now, with that third child, and without it. What do those lives look like? What are the joys and challenges of each?
And secondly, what would need to happen before you would be ready to have a potential third child - is it getting into a better financial position? A certain career progression? Moving to a new area/bigger house? Older kids at primary/secondary school?
If you come to the conclusion that a third child is something you really want, of the factors currently preventing you from taking that step, can any of them be accelerated? Or can any be delayed until after the arrival of a third dc? E.g. If it’s an issue of house size, is there a way you could make it work in your current house with three kids?
None of this is ideal, obviously - but some families do have to speed up decisions about having dc for medical reasons.
Maybe it’s a case of having that third child earlier than planned, so that you can get the medical care you need in order to be the best mum to all your kids?