How long does it takes cats to learn their names?

I can’t even remember when my 2.5 year old learned her name, but she definitely knows it. It’s not just inflection either, because you can test her by saying a series of names, all in the same inflection, i.e., “Laura? Stephanie? Kelly? Ilsa?” and she won’t look at you until you say Ilsa. She also responds to Ilse, Ilsie, Ilsagirl, Ilsababy, and Ilsa Margaret when she’s in big trouble.

We’ve had our two kittens for almost 3 months now (they were five weeks when we took them in), and it seems they still don’t know their names (Nicolette and Alexandria). It was easy to teach Ilsa her name because she was the only one around, so there was no room for confusion, but I’m worried that Nicki and Alex are having trouble learning their names because they’re not realizing which is being called. Additionally, we use many nicknames for them (Nicolette, Nicki, Knick-knack, Nick, Nickigirl, Nickibaby, Nicolette Grace when in trouble; Alexandria, Alex, Allie, Alliecat, Alliegirl, Alliebaby, Alexandria Rose when in trouble).

Why haven’t they learned their names yet, and how can we reinforce their identities?

If it helps, here are their photos:
Ilsa Margaret
Nicolette Grace
Alexandria Rose