I can't comment on the loss of your dear baby, I'm so sorry.
I can tell you about having 3 children, though. We thought DD1 had additional needs from an early age, but were told we were being paranoid. Specifically me. I'm a nurse - they thought I was medicalising infanthood. We had DD2 when DD1 was 20 months old. She seemed like a genius compared to DD1 - so advanced! I don't mean that negatively, but DD1 didn't really 'play' with toys, and DD2 lined things up beautifully and moved trains around a track. That sort of thing. So we decided to have DD3.
I was 5 weeks pregnant with DD3 when preschool 'asked for a chat' and 13 weeks pregnant when DD1 was admitted to hospital for tests. She was seen over the course of a few months. Fragile X, chromosome screen, CT & EEG, development tests, etc. Epilepsy diagnosis and global development delay.
Finally, when DD3 was 4 weeks old, we were told that DD1 has a brain malformation, which is responsible for her delays, etc. She had full 1:1 at preschool and then moved to special school. Now, aged 15, she's on the waiting list for ASD assessment.
Meanwhile, DD2 really struggled at school. She ended up leaving 2 schools, then home educated, then attended a 3rd primary. First school thought I was fabricating and inducing illness. Age 11, just at Secondary school, DD2 was diagnosed with ASD.
Life with 3 under 3½ was very hard. DD1 didn't walk until almost 2, so I had 2 non-walkers. Her sense of danger didn't develop and she has very poor impulse control. I couldn't safely go out with the 3 children alone. I still had to, of course, but it was very unsafe at times. We had to use a Crelling harness for DD1.
Life at home was so difficult that we had help from Home Start and SS also provided support 2x3 hours each week, so that I had an extra pair of hands before DH came in from work.
Now, they are 15, 13, 11. I love them all deeply, but their needs often conflict. DD1 can be impulsive and chaotic. DD3 has to share a room with her and is very particular, neat and fussy, so not an ideal match. DD2 is extremely introverted and needs lots of quiet space.
I would, in your situation, stick with 2 children. Another baby won't replace the one you lost and might tip you into an unmanageable situation, and that's before you even start to contemplate your new baby having SN