I hope it all goes as well and comfortably as possible, especially with going for three under three. It might help talking to a medical professional soon about getting a full nutritional screening - close pregnancies raises the risks of nutritional gaps even if taking vitamins and low levels of Vitamin D and some other nutritional needs can make obsessive thoughts and emotional lows worse. Getting a full screening and a chat with a nurse practitioner about it made a world of difference.
While you're emotions on this are running high at the moment, and will likely, hopefully pass. How to handle the disappointment depends on how you generally handle disappointments. I know some mothers where it was barely a flicker at a scan, quickly passed without any further thought; others who - having been pretty wrapped up and convinced of an idea - it took a bit more focusing on the positives and joys and working on not ruminating on what they think they might have missed.
It's normal, human, to have dreams and hopes -- and as adults, we realise that reality is never going to be exactly like we imagine so it's better not to get too stuck into our imaginations, as appealing as they can be.
I also know mothers who never got over it. I understand why some have reacted so firmly - this mindset can have serious consequences. I'm a child whose mother never got over what we now call gender disappointment. I can advise that her solution and the solution of mothers of her time that I knew -- openly discussing her disappointments in front of their kids, strongly dividing 'boy activities' and 'girl activities', hyper-masculinity and hyper-femininity as ideals, a lot of conversations around what boys and girls should do & picking at faults, and self medicating did not work well by any measure. It was very common at the time, and may be why more than a few are sore about it.
My mother and her mother were very close, up to her my grandmother's death. I was close with my grandmother, but I never got to have that with my mother, because I was, am, and always will be my mother's disappointment - not the son she was convinced she was having and not the kind of daughter she could love.
But are you honestly trying to say that boys and girls have the same amount of energy? Because if so, it’s scientifically proven that they do not. And that’s down to hormones (testosterone, estrogen etc).
There are various scientific suggestions on averages, but that means little to individuals and really, it's goes far beyond sex hormones - all of our hormones work together in that.
I've two of each sex. My sons will quite merrily sit most of the day, and did since being small. My older daughter cannot get through a day happily unless she can spend a significant amount of time stomp-pacing in peace - she's autistic and it's how she decompresses. My younger daughter volunteers as a farmhand and was the baby who would not stop kicking during the night to the point we had to move her to a bed far sooner than any other child.
My brother was the calm, placid, very academic type pushed into sports & treated like a disappointment for not excelling at that. I was the nervous energy type pushed to be more graceful, more performative & was a major disappointment at that. It's really not helped to push the 'scientifically backed' way we're supposed to be anymore than the cultural way we're support to be, especially when working through accepting the reality that your next child might not be how you're dreaming of.