would and could a docsis 3.0 Motorola sb6141 cable modem go from 3 channels upstream to 1 channel if it had to?
Yes. It can dynamically fall back from bonding multiple channels if conditions such as noise ingress on the lines is interfering with normal operation.
Or if signal levels get too high. 3+ channels = 51dB Max. 2 channels = 54dB max. 1 channel = 58dB Max (max based on 64QAM. 16QAM can be 1dB higher.)