It's just that Telegram is the most used one in Russia so they proceed by targeting this one first but don't worry they will target the other ones after.
Not even close. Whatsapp and Viber are much more popular. Telegram was barely 3rd before the ban. [1]
Viber moved their data to Russia, they won't get a ban.
Not sure about Whatsapp.
[1]: https://leonardo.osnova.io/bf345b20-423b-c508-cbc2-15963a7af...
Good catch!