We recently released a change to the way that Campaigns function, making them open ended. This means that subscribers will continue through the series when new messages are added to the end of it. This will happen even if the subscriber reached a point that was previously the final action in the campaign.
Because campaigns are open ended by default, subscribers will receive all messages in a campaign, regardless of whether the message was in the campaign at the time the subscriber began it.
For example, if a subscriber has received all 3 messages in a campaign, they will receive the next message you add to the campaign. This will happen no matter how long you wait before adding that next message to the campaign.
If, however, you add a message between messages 1 and 2, that newly added message would not be sent to the subscriber because the subscriber passed that section of the campaign already. The subscriber would not receive any new messages from the campaign until a message has been added after the messages the subscriber has already received.