The shifting mechanism is different between the 3-speed Nexus and hubs with more gears. So you might be able to swap them out, but you’ll have to check your bike frame to see how the gear-change mechanism mounts to the axle and rear frame chainstays. Shimano also makes a variety of different Nexus models with different widths, so you would have to make sure you get a Nexus 7 model with the same hub width as your Nexus 3. Depending on the age of your bike, the correct Nexus 7 hub might be hard to find. Newer “Alfine” models have a totally different shifting mechanism that might not be compatible with your bike - you’d have to check your rear frame to see.