A family of Microsoft word processing software products for creating web, email, and print documents.
The 'proper' way to do this is kind of labour intensive and involves editing field codes. If you are up to the challenge your fields should look like this
Caption 1a -> {SEQ Figure \* ARABIC }{SEQ FigX \* alphabetic }
Caption 1b -> {SEQ Figure \* ARABIC \c }{SEQ FigX \* alphabetic }
Caption 1c -> {SEQ Figure \* ARABIC \c }{SEQ FigX \* alphabetic }
Caption 2 -> {SEQ Figure \* ARABIC }
The \c switch tells the sequence to repeat the last caption number.