That’s how I feel unfulfilled. I love my boys dearly but it’s so hard not having kisses or cuddles or hearing the words I love you.
That's a lot of expectation to put on the third child. My advice would be to have some therapy first to be sure you're not having a child to fulfil your own unmet emotional needs as that would be a huge amount of pressure for that child if that's the role they're being born into.
It's also worth remembering that the experience of family life that you feel is lacking is what you're bringing this potential other child into - they're going to experience never receiving that affection from their older siblings too.
I have 3 disabled DC (ASD, etc etc) who are now teens. Although I'm their official carer and the DC don't have to do any of the active care, they're effectively carers for each other in lots of ways too as they have to modify their own behaviour and their expectations of life to accommodate their siblings' needs and are restricted in many ways by factors beyond their control. It's unfair but it's just how life is for them, in the same way it's just how life is for me.
It's also worth remembering that even if another DC isn't born with SN, they might develop MH issues from the pressure of what sounds like an extremely challenging situation. Hopefully if you have another child they'll be fortunate enough to be incredibly resilient and to cope with everything life throws at them. But what if they're not?
Having regard for a SN family member at all times and accepting that your needs will always be secondary to theirs is fucking hard for a parent. For a sibling without SN it's utter shit. And for a sibling with SN, it can prove impossible.
Do what you have to do. But be clear as to why, as the child will likely grow up and ask you if things don't go well for them.