So, in older versions of KSP, I had no problems showing 3 crew portraits in the bottom right corner of the screen (as seen in the several-years-old screenshot below). If I recall correctly, it showed 3 slots automatically (and maybe 4? I could be wrong about that).
But nowadays, I only seem to be able to show 2 crew portraits, as seen below. I can't add more portraits, as the "Add Slot" button is dark-grayed out and unclickable when 2 portraits are already displayed.
I tried scaling down the crew portraits so there would be more room on the screen, and maybe the game would let me add more...but no such luck. If anyone knows how to add more slots, or if anyone knows of a mod that would accomplish this, I'd love to hear. It just bums me out to have the command pod with 3 seats but I can only see 2 Kerbals...!