To the OP, I highly doubt that the gripper you almost closed was a COC #3. I’ve only heard of 1 or 2 people who ever closed the #3 the first time they tried it.
By, the way, just to give everyone looking for a feat of grip strength to aspire to. One of the people who I know for a fact closed the #3 the very first time he tried it is one of my martial arts instructors, Charlie Lysak.
He also has worked up to being able to close the #3 100 consecutive times (no that’s not a typo). Unfortunately, doing such a high volume of intense grip work caused him to get carpal tunnel and he had to back off the grip work for a while. He can still close the #3 every time he tries though, and is damn close to closing the #4.
As far as the grippers, I can close the #1 and #2 with either hand, but I’ve only see one other guy at the gym where I train close the #2 (most can’t even close the #1) and he is a former division 1 football player.
As for advice to being able to close them, I second negatives. However, honestly what got me that last 1/2 - 1/8 inch was just trying to close them once every hour for a couple weeks. I would just give it everything I had for a couple seconds (not until failure) once every hour (which worked out to around 8-10 attempts per day) 5-6 days per week for a couple weeks.
Eventually I was able to close the gripper with either hand. I repeated the process once again once I was close enough with the #2.
I’ve got the #3, but I haven’t worked with the #2 enough lately to start trying this with the #3 yet.