No, Realme 3i does not support fast charging. Though it does not support fast charging still it won't take much time to charge your phone. You can expect your phone to be fully charged in 2-3 hours.
For complete specs: Realme 3i
Yes, Realme 3i supports VoLTE in both the SIM slots since VoLTE works on bands 3,5 and 40 and both the SIM slots in Realme 3i supports band 3 and 40. You can use two Jio SIMs at a time in Realme 3i.
You can check the specs of Realme 3i here: Realme 3i