Hi S.-
I am the youngest of three, I have an older brother and older sister. We are a combined 3 years apart (my brother is 18 months older than my sister and she is 18 months older than me). I always said if I ever had kids I would never want an only child, but after having a baby at 19, I have only one child and wish he had the experience of siblings (don't know if that will ever happen and if it does it will be very different for him). We were a tight nit group of siblings and still are.
I can tell you as the youngest I got ganged up on by my brother and sister quite a bit, they picked on me a lot, but it didn't change the fact that we loved each other dearly and most of my best memories as a little kid revolve around my brother and sister and how much fun we used to have together, playing in the woods behind our house, snowball fights at grandma's, picking berries by the train tracks, playing in the creek, riding our bikes, etc. I think getting picked on then only made me more resilient later!
I can also say that I think because my sister and I are girls my brother started to feel more left out later as teenagers, but that passes just like the normal awkwardness of teenagedom. Every kid is different, we all three have very different personalities, my brother is the genius of the family, a quiet thinker, while I'm the vocal, lots of laughs, more brazen one, and my sister is the hardheaded, do things my own way type. Different dynamics will play out differently dependent on personalities and you can't control any of that, but as long as you continue to keep them in a nurturing environment, make them play together (my parents always made my siblings let me tag along) they will all grow up loving each other and knowing they have each other to count on no matter what!