how to convert shift-jis, cp1250,etd. id3 tags to utf8?

You can try Mp3tag with its 'Convert codepage' action type for that. Select the file, create a new action via 'Converter > Actions' and choose the codepage you want to convert from the drop-down list.

Best regards,
~ Florian

In Linux you can use EasyTag (but it can save only to 16 bit Unicode, not to UTF-8).

Btw, the 'Convert codepage' action is only needed, if your tags are displayed wrong in Mp3tag. Otherwise you only have to save the tags again to convert them to UTF-16.