Hello...
I have three grown children, 24, 21 (within days of turning 21), and 18. 3 is not such an odd number.
My children get along fine-even over the rougher spots, and there were some, but they pass easily. They are supportive of one another very much and very very encouraging of one another. And they, all three, are close. Sometimes two of the three get together sure, but that never has been an issue of one being left out, but one of availability.
My oldest is more serious, my daughter in the middle is getting to be that way so is more drawn to her older brother than ever, and the youngest supports them both in their growth and pursues his own way...which is also beginning to get more serious. He works with my oldest, so they have their own space that is where their intimacy happens that they share. And the youngest respects the oldest very much.
It's all appropriate for all of them. We have always stayed out of their relationships with one another and trusted them to learn what worked and we respected that. Easier said than done but practice makes perfect.
See things really even out even when there is a rough spot or two because they can't help but learn from each other over the years. If parents learn to allow and let the children lead the way...without second-guessing and so on, there is room for growth and fun and natural progression through life. Our family has very little if any friction in it. We enjoy alot of relaxation together and it is really wonderful.
My oldest paved the way for the others in school, in growth, in life. They love him and he loves them. But he had his friends and spent alot of time with them...staying overnight when appropriate, or hanging out with them after school. He was the guy everyone wanted to push them on the swings because he was/is big and strong. Everyone likes/loves him...and he is very generous and very loyal, which is a big part of why his siblings love him, admire him, and trust him so very much.
They all have their own way of being too, so I think actually....it comes down to personality and so on....and is not anything we should feel weird about. My kids worked it all out from the beginning.
For instance, my two boys (the 24 and the 18 yr. old) played together alot...they built stuff together. My daughter was never interested in that at all. She loved books, doing beadwork, playing dolls, studying and spent alot of time with her girlfriends at their houses. She had her own way.
While my daughter, played with her dolls...my oldest also played with her with dolls in a nice "daddy" way because of witnessing the example of his dad being a tender kind of father with the youngest, but also enjoyed playing with tools and doing construction with his younger brother when that time came.
See? they really are ok as 3.
They all played together as kids and swam together and listened to books together and read to one another. I used to do alot of walks with them, I did visualizations with them, and we did alot of art together. I was always a SAHM....and a bigger kid than any of them, enjoying them so much.
They were each far enough apart to develop their own way and their own friendships from early on, so their diverse social paths eliminated the whole "left out" thing. But they enjoyed being together too.
Just here to say that 3 kids is wonderful and fun to watch growing up. I love my 3.
Sure we thought of having 4, but really...by then the others were pretty grown and a fourth would have been like an only child. Not that there is anything wrong with that, but it just didn't work for us as we considered it.
So there you have it from a mom of 3. I hope that something I have shared here, helped with your discomfort or shed light at least on the potential realities of 3 kids.
By the way....my personal experience of being one of two children: My only sister died when I was 17. She was 10 years older than I. When she passed on...my parents withdrew from me very naturally, and turned to comfort one another. I was the odd man out and like I said, it was not their fault. Turning toward one another is a very natural thing in my mind. But for me, when I had two kids, a boy and a girl...I had to consider what might happen if one of them were to (gods forbid) pass away...and I decided that even numbers allowed someone to be left hanging in that scenario.
We all have reasons for choosing the number of kids we have...including that it just kinda happened that way.

Oh and let me add, that my kids suffered personal losses in the last few years: first in '99 they lost their grandfather and were present when he passed and they really helped one another out and helped us too. And a couple years ago...they lost a friend they had in common (small town), who was in fact a friend of all of us, a very young man (my daughter was in love with him for a long time actually)...and they not only helped to comfort one another, but reached out to the community to help others too. It was a proud moment for me to see how compassionate they all were to one another and to others.
Children just learn what they live I s'pose.
May your family grow to be loving, strong and present for one another.
Blessings...Joyce in the mts.