Gible - None (PokeTransfer.)
Dratini - Fishing at the entrance of Dragonspiral Tower.
Bagon - White Forest (You must bring Grace From Black City.)
Gible is the only one out of those three that you cant catch in Black/White. Dratini can be caught at Dragonspiral Tower and Bagon can be caught in White Forest.But next time just look it up
Source-Here and Here