The heart of TripSit is and will be IRC, which we deem a secure and trustworthy platform.
Matrix is a secure communication protocol which we decided to bridge to selected channels in a closed beta for regular and trusted users to try out. If the reception stays good, further channels will follow. If you have been around TripSit for a while already, feel free to ask in #tripsit-dev (if you've been invited already) or in #content to be granted access.
Discord is prone to privacy concerns, not only from IRC veterans, but from regular internet users as well. Because of this, we decided on only bridging one channel for general talk (#welcome) and one channel for non-IRC users to receive help in (#tripsit3) to Discord, and will refer users there to our Web-Chat for them to join further channels on IRC.