I suspect most of the imbalance is from the external wire bridges that connect the two channels. DP832 has compenstion wires internally tied to the output connectors. Also, there are some GND bridging issues with the GND bridged internally, between CH2 and CH3, not sure if those are supposed to affect your setup.
What I would try:
- make sure the bridges are firmly tight and making good contact, and that the wires are made of solid copper and not just copper plated
- tie together all the GND connectors, including the GND from the 3rd channel, and including the earth grounding banana
- connect both + and - wires to the bananas of a single channel, try either CH1 or CH2 to see in which channel the current gets a better distribution
- If you have 2 pairs of wires for the load, use 2 pairs instead of just one and connect each pair to a channel
Could also be a wrong calibration in the DP832, but that would be highly unlikely, verify the bridges first, and use 4 wires to the load instead of only 2, if possible.
Another way to raise a little the total current would be to set the max current to 3.2A on each channel, so instead of 3.0+1.7 max current, you'll get 3.2 + 1.9, so instead of only 4.7A you'll get 5.1A in total.