[user104658]

Oh also yes the TARDIS is overstuffed with 3 companions, it just didn't work. Two seems to be the upper limit. Doctor-Rose-Jack was fine as dynamic, as was Doctor-Amy-Rory. And even Capaldoctor-Bill-Nardole was OK although its not my fav era.

Although in each case a duo is established BEFORE adding the third wheel.

You can also have an occasional extra member in the mix. River Song, Missy etc... But a permanent ensemble cast is too much for a show that at the end of the day revolves around one central character. Takes too much of the focus.