Is this how the Block Meter works?

It is totally random. You have an equal chance (1/3) to get each option (fill 1, 2 or 3 sections) when you are going to suffer damage.

I've been playing this game for a pretty long time, and I have never seen anything to make me believe otherwise.

I've been in games in where every single attack filled me just 1 section of my block meter, until like, turn 6 or 7.

I've been in games in where every single attack filled my block meter 3 sections, getting all my superpowers by turn 3.

There was a thing long time ago, in where you had a 5% chance to instantly fill your Block Meter. It was called a Insta-Block (or Mega Charges), but the devs got rid of this.

So, yeah. It is just bad luck.

In the end, you can believe whatever you want, but I encourage you to try an experiment.

Write down every time you got a section filled to your block meter, and make a list.

Something like...   1 section, 1 section, 3, 2, 2, 1 - game

                               3 sections, 3 sections, 1, 3, 2, 1, 2 - game

etc

Write down 50 samples, and compare results. You should get something near 33% chance for each result. 

(Since it is completely random, you can get something like... 35% chance for the 1 section, or 30% for 3 sections, etc. But it should never be something too far from that, like 70% for 1 section)