Actually wrong.
Tuner A dvb-S2
Tuner B dvb-S2
Tuner C DVB-T2 or DVB-C
All can operate at the same time. So 2 Sat-s plus DVB-T2 (or DVB-C).
If I have 2x Triple tuners, then at the same time 4x Sat plus DVB-T2 (or DVB-C) plus DVBT2 (or DVB-C).
So it is with dreambox Triple tuners.
I am pretty sure that SI2169 can handle only 2 TS streams and 3 tuners are connect (2x dvb-s2 + 1x dvbct2)
Like you inform you say you have three unique tuners and so NO multituner.
So your info is wrong or DM920 has a bug as it suggest it has a multituner.
Single multituner is
Tuner A dvb-s2 / dvb-c / dvb-ct2 / atsc
Dual multituner is
Tuner A dvb-s2 / dvb-c / dvb-ct2 / atsc
Tuner A dvb-s2 / dvb-c / dvb-ct2 / atsc
Tripple Multituner is
Tuner A dvb-s2 / dvb-c / dvb-ct2 / atsc
Tuner B dvb-s2 / dvb-c / dvb-ct2 / atsc
Tuner C dvb-s2 / dvb-c / dvb-ct2 / atsc