Yes, you can add an 8GB SODIMM, and it will partially run in dual-channel.

Your Inspiron 15 3000 typically has one soldered 4GB or 8GB stick and one open SODIMM slot. Adding an 8GB stick gives you 16GB total. The first 8GB (the soldered portion) will run in dual-channel with the matching 8GB of the new stick. The remaining 8GB from the SODIMM (the extra capacity that doesn’t have a matching channel partner) runs in single-channel. So you get a hybrid mode — better than 8GB single-channel, but not full 16GB dual-channel.

Just make sure you buy a DDR4 SO-DIMM that matches the speed (likely 2666MHz or 3200MHz, check with CPU-Z or the Dell spec sheet). Avoid mixing different speeds or voltages — it’ll likely downclock to the slowest, but that’s fine. Future you will appreciate the extra RAM for Chrome tabs and Lightroom browsing.