Wrong. Both WhatsApp and Signal group chats are E2EE.

Telegram group chats are not. Even 1on1 chats aren‘t E2EE on Telegram by default.

Also, reporting is an issue: If a member of the group "Reports" a message to WhatsApp, a copy of the recent messages in that chat is decrypted and sent to WhatsApp for review to check for terms-of-service violations.