Here is a link to part 3.
One of the great things about Slack is that it serves as a direct IM tool, a more general discussion tool, and allows for private groups and channels.
When you're building a bot for this type of tool you need to think about where you want your bot listening and how you want it to behave when you are messaging directly vs. in a public channel.
For our first bot we decided that the main focus would be on the direct messaging and we would keep the bot chatter out of the channels (public discussions) and groups (private discussions). But in the event that it appeared that contextually someone should use the bot, we would make a suggestion asking if they wanted to direct message the bot.
We can then invite the bot to channels where we think this would be appropriate, and not invite the bot where we think it would be annoying.
Slack is quite powerful and well thought out in this regard and this type of cross-linking between messages, channels, and users is easy to do.
If you are just getting started with this, it is very helpful to read the Message Formatting Spec for Slack.