As long as your live rock is established and cured, it is that easy. The LR is your filter and cycles the tank instantly. The only catch is there will be die off on the rock in transport, so I would wait a couple weeks before adding any inverts. I would do biweekly water changes of around 20-30% for inverts only. Personally, I wouldn’t add any fish in a 3g. You could also do Xenia or green star polyps. But they will take over in no time, so you’ll have to do some grooming with those. Also make sure the LR you get doesn’t have any aptasia, that is nearly impossible to get rid of. I also recommend buying premixed RODI from your LFS for this size tank instead of mixing