I'd say lucky you! Your dive bag will always be 2-3 kg lighter than those of your friends. Flying somewhere, or hiking to remote diving locations is easier for you. So many advantages, without any disadvantages!
A friend of mine is exactly the opposite. For his 5mm he needs 12kg to be neutral at 8-10m below.
The rule from an AIDA instructor is :
In the water, upright, without moving, exhaling fully so in this case, the mouth should still be out of the water.
If you sink, you are too ballasted
If you don't sink in as you breathe out, you are underweight.
I liked to play with my buoyancy even before I started diving. I thought that I float so well in the sea, the water level is between my nose and my eyes when I'm upright in full lang, and I have to blow out about half of my air to start sinking.
In my shore dive lesson the instructor told me to try 1 kg first and when I reached -10 m, oh hell I was falling so fast that I couldn't even stay at the level for a second, and when I tried 0 kg I thought I was balanced. I thought I would need a lot of weights if I put on a wetsuit then.
In my boat dive lesson as the water was cooler I put on my 3 mm wetsuit. The instructor told me to put on 3 kg first, then 1 kg. I did the check more carefully and the instructor still saw me sinking at -10 m. Then he told me to remove the weight belt, and I got to -10 m again. The instructor then told me I was alright there. I was so damn surprised.
I'm thinking about what will happen if I dive in fresh water instead of the ocean without a suit and weights. In such case my neutral point will only be about -4 m even in full lung, the water level is just a few cm from my top upright at the surface without moving, and I sink immediately if I let go about 1/4 - 1/3 of my air.
