I was making my new era, then realized all the letters for eras had been taken. Is there a way to use numbers? I got it to work multiple times with letters, but never the number 3.

Look in the patch 1.3 docks. Here's the relevent info anyway:

I saw that, but whenever I added a letter era, it worked, but when I replaced the letters with numbers, it didn't show up.

The number 3 isn't in that list, so if that's the only number you tried it won't work.

Could I use 4 then, or does it have to be a letter on that list? I'm wondering because all those are localized for different things, and as far as I know, not for what I'm intending.

It has to be on the list.

Okay. I went through the letters on the list, and z doesn't have a name or icon, so I'll use that. A final question, how would I add the name and icon to it?

You didn't understand the PMs. The only usable era characters are the ones posted above. If anymore where to be added, another revision of the v1.3 patch would have to be released. Meaning you would have to get everyone who downloads your map to redownload and reinstall the v1.3 patch.

However, you can rename any era for your maps (and each map can rename the same era character to something different). The v1.3 patch includes changes such that the era character really doesn't matter any more. Included in r117's docs is a guide on how to rename the eras and/or game modes your mod map uses.

1) Pick any of the above listed era characters
2) Use that era in your map
3) Follow the v1.3 patch's docs and rename that era for your map through the map's addme.lua

I thought that doc only said how to name a mode. Thanks, I'll check it out.
EDIT: where do I put my custom icon?