WhatsApp is an instant messaging service that allows users to send text and voice messages, make voice and video calls, as well as share various forms of media like images, videos, geolocations, documents, etc.
Telegram is a multi-platform, free to download messaging app that focuses on encryption, privacy, and an open-source API. It lets users call their contacts, create group chats, and send media files and stickers.
Messenger is an app that allows users to keep in touch with each other using messages with links and media files, as well as search for other users and groups. Developed by Facebook, Messenger has over a billion monthly active users.