We've ported the SDDM theme to PC3 and now have black labels in PC3 ToolButtons (keyboard and session button).
The SDDM theme uses a complementary color scheme, which is something the PC2 ToolButton respects and turns the labels white.
Therefore I just copy pasted PC2's label color code to its PC3 counterpart: https://github.com/KDE/plasma-framework/blob/master/src/declarativeimports/plasmastyle/ToolButtonStyle.qml