I believe this will technically be possible, but only when tuning for the physical channel and not filtering the stream. Unfortunately, this is not presently how Channels handles tuning devices that can accept the HTTP API (HDHR4 and newer).
As an example, say that you receive your NBC and its subchannels on 4.1, 4.2, and 4.3, and they are broadcast on UHF channel 32. The way Channels currently works is that each virtual channel is separately requested, and consumes a separate tuner. So, when you want 4.1, Channels asks the tuner for it, the tuner tunes physical channel 32, filters out all of the other subchannels, and delivers only 4.1. If you want to tune 4.3 on a different client concurrently, Channels makes a second request for 4.3, and it uses a second tuner on the device. (This is also how SiliconDust's software works.)
Another way of achieving this is that Channels would request the full physical channel 32 and receive that from the tuner. Then Channels asks the tuner to filter out unwanted subchannels, leaving only 4.1. Later if you additionally wanted to simultaneously view 4.3, Channels could change the filter to also include that subchannel. Channels would then handle distributing the filtered streams (a mixed stream with 2 muxes, which Channels then must separate and feed separately to the different clients). This is the way some other DVR software works, like Tvheadend.
ATSC3 allows for up to 4 subchannels on each physical channel. So, while technically Channels could handle the situation in the second manner described, it's unlikely that it will. The process will probably continue much as before, as in the first situation described.
(Bandwidth is also a concern. The HDHR5-4K will only have a 100Mbps adapter, while physical ATSC3 channels can have a maximum of 57Mbps. So, it's easy to see that by using the second scenario of having Channels filter the muxes could easily overwhelm its network adapter. ATSC1 has a maximum bandwidth of 19Mbps for each physical channel; this means that is all 4 tuners we're delivering full bandwidth channels, that would be just under 160Mbps.)