My youngest three are 8, 6 and 4.
Yes sometimes, but it's not always the same two that play together, and very often all 3 play together, so the negatives in that respect are far out weighed by the positives.
Mine are DS, DD and DD.
I think if you only had 2 DCs, there would be times when they didn't want to play together, but obviously this would never be seen as being left out.
When there are three, and two are playing together, it is not necessarily the case that one is deliberately being excluded....it might be that the game two are playing only appeals to two of them, or one just wants to play by themselves/do something different, IYSWIM.
Any deliberate exclusion usually occurs when DDs start a game together, DS hears them playing, asks to play and is told "NO!"
If that happens I say they either all play or none of them play! Not sure if that's good parenting or not, but it's what I do! 