Can I add an 8GB SODIMM to my Inspiron 15 3000, and will it run in dual-channel?

Yes, adding an 8GB SODIMM gives you 16GB total, but only the first 8GB (the soldered portion) runs in dual-channel with the matching 8GB of the new stick. The remaining 8GB from the SODIMM runs in single-channel, creating a hybrid mode that’s better than 8GB single-channel but not full 16GB dual-channel. Just buy a matching DDR4 SO-DIMM (likely 2666MHz or 3200MHz) and avoid mixing speeds or voltages.