They are pretty different just in capacity. The iclear 30s is a 3ml tank, the mini vivi nova is 2.5 ml, and the mini PT3 is a 1.5 ml tank. I'm not sure what Nautilus you were looking at - they also come in a variety of sizes. Aside from how much juice capacity you want you might also consider warm vs cool vape. Top fed clearomizers like the iclear 30s will generally give you a warmer vape since the coils are physically closer to the tip. The mini VN works in the same way, but I believe it's just a single coil instead of the iclear 30s' dual coil setup. Hence you could expect it to provide a bit less vapor generally speaking (I don't have any personal experience with the mini VN). The mini PT3 is fed from the bottom; hence you might find the vapor to be more cool than in a top fed tank. The mini PT3 also has dual coils like the iclear 30s.
You also should consider if your battery is a good fit power/capacity wise and also aesthetically if that's important to you. For the dual coil tanks you are looking at I would suggest having a variable voltage device so you can boost up the power if you feel you're not getting enough power into the coils to properly heat them. If you have a VV battery that doesn't look weird with a large tank I would go with the iclear 30s. I've not heard anyone say much against these and I'm thinking of getting one myself since I tend to like the warm vapor and big flavor I get from my smaller wick fed tanks.
Good luck and happy
vaping.