The key of Eb has Bb Eb and Ab. G is the third degree in the scale.. no accidentals to alter it. Just a regular major 3rd over the tonic.
In the key of D (has F# and C#), the major third over the tonic would be F# -- that's right. You wouldn't add any other accidental to alter it, you just want to keep the normal major third... it's not an altered note, so it's just the note specified in the key signature. Because of the key signature, by the way, you don't need to write the # symbol by the note.
-Rob