I have the same ages as you do, but girl, boy, girl. The little one is left out a good amount, but not that much. My oldest daughter likes to mother her and when she has friends over, they mother her together. But my middle son doesn't love to play with her as much and I'm guessing if my oldest was a boy, she would be left out a lot more.

What if two of the three are twins.

This may get better once the youngest can really play. Can you create a whole-family activity? Ours was snow skiing growing up but it could be anything- hiking, Star Wars movies, anything - to encourage interaction.

My 3 are all around 3 years apart. G, B, B. Two of them often play together while the other does something independently or with me, but who plays with whom is constantly shifting. They kind of appreciate each other for what they each like to do. If middle boy wants more physical play, he’ll go find his brother and go outside. If he wants a board game or to watch silly YouTube, he will find his sister. Oldest and youngest are very cute together. The youngest is kind of the pet of oldest and her friends. They carry him around and generally baby him.

That’s the age range, OP. If you had two kids five years apart, you’d still likely have these issues. Our three are 2 years, and then 2.5 years apart, and while the older two play together most often, no one kid is more excluded than the others. They switch pairings and each is happy playing independently, so it works well so far (all are young, 7, 5, and 3, G, B, B).

Sorry, so your kids are like Boy 9, Boy 7 and Girl 2? Then of course the little one is left out. Honestly, with those gaps/ages, that's more an age thing than anything else... though gender probably doesn't help. 7 and 5 years are big gaps, the older siblings will be more like close uncles than brothers relationship-wise until they are much older. That's OK though.

The kids in my family are spaced 11B, 10G, 6G, 1B and, yeah, my older brother and I (I am the 10G) did not have either 6G or 1B at our playdates when we hung out w/ our friends. We played with our friends and 6G and 1B hung out w/ our parents (or 6G had her own friends over). Probably as a combo of personality and spacing, 6G and I were never close, though we get along fine now; 1B and I are actually reasonably close as adults (though our relationship is still a cross b/t sibs and aunt/nephew); and 11B and I remain and have always been super close.