That character mapping most likely is not defined in that Romans font.
If this is MText (or dim text) you can type in the numeral "3", select it, and then use the Text Formatting toolbar to force it to superscript:
Hi,
>> Why font romans dont show cube (³)?
Because romans.shx is a simple, legacy SHX font that does not support Unicode.
The cube symbol (³), is not included in the character set of the romans.shx font. This is a limitation of the font format, not a bug in AutoCAD.
The only way for that is to use a Unicode-compatible font (Arial,Calibri... )
Imad Habash