I have a Major system for 00-99, and I use it for pairs of digits like 8297 (a vine growing around a book) or 5873 (a lava game… hmm…), and when I need to memorize a 3-digit number that I don’t have handy, there’s a lone digit that doesn’t get accounted for.
In practice, many people use a raw 0-9 list for the missing digit, but often if the number is 3 digits long, I might incorporate that digit into my image, either by thinking up a number, like 420 is Uranus. But as a beginner, I don’t have the time to come up with and memorize 1000 images.
Instead, I will take the first digit and make it an adjective. I had images for 2, 3, and 4 which I use the most often, but I decided to extend it for the other digits. If I don’t have an image for a 3-digit number, this is my temporary solution until I can think of one.
0 - icy
1 - hot
2 - new
3 - my
4 - your
5 - ill/oily
6 - itchy/which (I look around and ask “which X?”)
7 - yucky
8 - heavy
9 - happy
So for memorizing 524, which I have no image of, I’ll think “oily honor”, which my image is of a Ph.D. stained with oil. Or for memorizing 928, I’ll think “happy nova”, which my image is of a star going supernova and laughing maniacally. Or for 754 - “yucky oiler”, I think of a monster eating one of the Edmonton Oilers (a hockey team), and think “ew”.
If anyone needs a temporary 000-999 system fast, maybe this will help. If you use something else, think of how you can apply the same theory there.