Never seen 3 x upsampling before. I wonder what this means?

Usually it's 1x, 2x, 4x and 8x.

Is there a spec sheet somewhere?

Streaming: Roon, NUC, QNAP869 NAS
2ch system: dCS Vivaldi, Audionet Stern, Audionet Heisenberg, Magico Q1
Retro system: 101/104/105, Red Rose Spirit