Todd Howard announced at Microsoft’s E3 conference that Xbox One users will be getting a copy of Fallout 3 with an Xbox One version of Fallout 4, and all mods available for PC will be available for Xbox One upon release.
Immediately after announcing backwards compatibility, Bethesda revealed that Fallout 4 players will exclusively receive Fallout 3 for Xbox One. It is unannounced whether or not the version of Fallout 3 will be a remastered edition or the Xbox 360 version.
Fallout 4 will exclusively receive all mods published for PC on Xbox One consoles. Whether or not the Xbox one version will receive mods for Fallout 3 or not was left unannounced.