This is really a difficult question since each country has a different law when it comes to this topic.
At first I would recommend you all to wipe your devices and re-setup them in order to avoid some shit like "Mr/Mrs xyz encouraged person 2 and person 3 to commit suicide together" or anything like that.
Also, don't send messages about where and when you all plan to commit suicide.
Use Keybase and create an account there with an throwaway mail.
Remember that Whatsapp, Telegram, IG, Snapchat and FB Messenger have cloud features and I don't know how Telegram handles it but for example Whatsapp saves messages for at least 30 days!!
If you need help with creating anon keybase account, feel free to PM me. I have a temporary mail provider that still works with Keybase