WhatsApp does more security than telegram, i don't know much about Discord but i make use of WhatsApp and the options of securing yourselves from unsupported conversations is free unlike telegram that demands you to use their premium feature. If i don't give out my number, no once can add me up, message me even through groups, i set my restrictions too well.
It is? Im not sure though but there are some red flags when I use whatsapp. Yeah telegram isnt safe either but I think its more used by many, I am using it due to crypto related stuff and communication to some airdrop projects. What I dont like about it its swarming with scammers dming you.