Unfortunately this still isn't present in Gwibber 3.0.0.1
Considering that most Twitter clients will (by default) reply to all users mentioned in a tweet, it would probably make sense to implement this in Gwibber. If you're having a conversation with multiple people, it isn't particularly convenient to have to remember to copy everyone else's names when you reply to a tweet.
Unfortunately this still isn't present in Gwibber 3.0.0.1
Considering that most Twitter clients will (by default) reply to all users mentioned in a tweet, it would probably make sense to implement this in Gwibber. If you're having a conversation with multiple people, it isn't particularly convenient to have to remember to copy everyone else's names when you reply to a tweet.