It's possible to use special characters in variables by first putting them in quotes, then deleting the quotes. However, I don't think this trick works to get a variable starting with a number. I just tried in P3.1 and it didn't work. Perhaps P4.0, but I doubt it.
The following special characters look like 3's and are accepted as the first character of a variable (quotes trick needed): З Ӡ Ʒ ʒ
I'm sure it's not what you were looking for, but the best work-around I could find.
The first is:
|U+0417||З||Cyrillic Capital Letter Ze|
However, it does not have a keystroke (Alt+Numpad) shortcut.
The others are similar.
I think the easiest thing to do is copy the character to your clipboard and paste as necessary.
Paste results in ? not 3.
First type quotation marks, then paste the З, then delete the quotation mark.
After that, I'd copy the variable name and paste it where needed in equations.
I also like Luke's solution. I was unaware of ALT+0173.