Let's make a list for full overview.
Bora
Giro: Majka / Tour: König / Vuelta: Majka
Astana
Giro: Aru / Tour: Fuglsang/Lopéz / Vuelta: Aru
BMC
Giro: Tejay? / Tour: Porte / Vuelta: ?
Sky
Giro: Landa or Poels / Tour: Froome / Vuelta: Landa or Poels or Froome
Trek
Giro: Mollema! / Tour: Contador / Vuelta: Likely Contador
Lotto-NL
Giro: Kruijswijk / Tour: Unknown (maybe neither Gesink or Kruijswijk) / Vuelta: Gesink?
Baharin
Giro: Nibali / Tour: ? (possibly Izagirre or Purito) / Vuelta: ? (possibly Purito or Izagirre)
Movistar
Giro: not known (could be Valverde) / Tour: Quintana / Vuelta: Valverde, Quintana or both
FDJ
Giro: Most likely Pinot / Tour: ? / Vuelta: ?
AG2R
Giro: ? (Pozzovivo or Frank?) / Tour: Bardet / Vuelta: ? (Latour?)
Etixx
Giro: Jungels again? / Tour: Martin / Vuelta: De La Cruz again?
Orica
Not decided yet other than Chaves and Yates Brothers are the leaders
Cannondale
The same as last year? ; Giro = Uran, Tour = Rolland, Vuelta = Talansky?
Orica
Not decided yet other than Chaves and Yates Brothers are the leaders
Giant
Giro: Big Tom? / Tour: Barguil / Vuelta: Barguil AND Big Tom?
TJ
Meintjes is likely to be Tour leader but is Costa going to target a GC?
Soudal
No real GT riders, Gallopin being the closest. Maybe he targets a top 10 spot in Tour De France?
Dimension
No serious GT riders, therefore no captains?
Katusha
I think Zakarin wants to give the Giro another go. No idea who will lead the Tour or Vuelta. Maybe Taaramäe for the Tour?
Sources:
http://www.denik.cz/ostatni_sport/konig-opousti-sky-vraci-se-do-tymu-bora-na-tour-2017-bude-lidrem-20160920.html
http://sport.tvp.pl/27258397/majka-w-2017-roku-chcialbym-pojechac-w-vuelcie-i-giro-ditalia
http://www.lequipe.fr/Cyclisme-sur-route/Actualites/Plutot-giro-et-vuelta-pour-fabio-aru-en-2017/739778
http://sport.tv2.dk/cykling/2016-10-18-jakob-fuglsang-alt-tyder-paa-jeg-bliver-tour-kaptajn
http://www.cyclingnews.com/news/porte-set-to-make-2017-debut-at-tour-down-under/
http://www.cyclingweekly.co.uk/news/racing/tour-de-france/froome-pleased-2017-tour-de-france-route-organisers-try-inspire-attacking-racing-290514
http://www.cyclingpro.net/spaziociclismo/transferts/ciclomercato-landa-rimane-in-sky
http://www.skysports.com/cycling/news/15264/10137127/team-skys-wout-poels-keen-to-lead-team-sky-at-future-grand-tour
https://www.procycling.nl/2016/10/19/bauke-mollema-2017/
http://www.cyclingnews.com/news/contador-confirms-he-will-target-the-tour-de-france-in-2017/
http://www.wielerrevue.nl/2016/07/11/kruijswijk-2017-twijfel-tour-alles-op-giro/
http://www.telegraaf.nl/telesport/26836487/__Gesink___Geen_klassement_in_Tour_2017___.html
http://www.cyclingquotes.com/news/nibali_hoping_to_target_100th_giro/
http://www.elespectador.com/deportes/ciclismo/podra-nairo-quintana-ganar-el-tour-2017-articulo-660904
http://www.cyclingquotes.com/news/pinot_in_2017_i_will_focus_on_the_giro/
http://www.nation.co.ke/sports/othersports/Froome-and-Bardet-eye-2017-Tour-de-France-mountain-duel/1951306-3421760-u3ssxlz/
http://www.cyclingnews.com/news/dan-martin-the-tour-de-france-is-where-its-at/
http://www.francetvsport.fr/videos/video-les-etapes-cles-du-tour-de-france-2017-363689
Bora
Giro: Majka / Tour: König / Vuelta: Majka
Astana
Giro: Aru / Tour: Fuglsang/Lopéz / Vuelta: Aru
BMC
Giro: Tejay? / Tour: Porte / Vuelta: ?
Sky
Giro: Landa or Poels / Tour: Froome / Vuelta: Landa or Poels or Froome
Trek
Giro: Mollema! / Tour: Contador / Vuelta: Likely Contador
Lotto-NL
Giro: Kruijswijk / Tour: Unknown (maybe neither Gesink or Kruijswijk) / Vuelta: Gesink?
Baharin
Giro: Nibali / Tour: ? (possibly Izagirre or Purito) / Vuelta: ? (possibly Purito or Izagirre)
Movistar
Giro: not known (could be Valverde) / Tour: Quintana / Vuelta: Valverde, Quintana or both
FDJ
Giro: Most likely Pinot / Tour: ? / Vuelta: ?
AG2R
Giro: ? (Pozzovivo or Frank?) / Tour: Bardet / Vuelta: ? (Latour?)
Etixx
Giro: Jungels again? / Tour: Martin / Vuelta: De La Cruz again?
Orica
Not decided yet other than Chaves and Yates Brothers are the leaders
Cannondale
The same as last year? ; Giro = Uran, Tour = Rolland, Vuelta = Talansky?
Orica
Not decided yet other than Chaves and Yates Brothers are the leaders
Giant
Giro: Big Tom? / Tour: Barguil / Vuelta: Barguil AND Big Tom?
TJ
Meintjes is likely to be Tour leader but is Costa going to target a GC?
Soudal
No real GT riders, Gallopin being the closest. Maybe he targets a top 10 spot in Tour De France?
Dimension
No serious GT riders, therefore no captains?
Katusha
I think Zakarin wants to give the Giro another go. No idea who will lead the Tour or Vuelta. Maybe Taaramäe for the Tour?
Sources:
http://www.denik.cz/ostatni_sport/konig-opousti-sky-vraci-se-do-tymu-bora-na-tour-2017-bude-lidrem-20160920.html
http://sport.tvp.pl/27258397/majka-w-2017-roku-chcialbym-pojechac-w-vuelcie-i-giro-ditalia
http://www.lequipe.fr/Cyclisme-sur-route/Actualites/Plutot-giro-et-vuelta-pour-fabio-aru-en-2017/739778
http://sport.tv2.dk/cykling/2016-10-18-jakob-fuglsang-alt-tyder-paa-jeg-bliver-tour-kaptajn
http://www.cyclingnews.com/news/porte-set-to-make-2017-debut-at-tour-down-under/
http://www.cyclingweekly.co.uk/news/racing/tour-de-france/froome-pleased-2017-tour-de-france-route-organisers-try-inspire-attacking-racing-290514
http://www.cyclingpro.net/spaziociclismo/transferts/ciclomercato-landa-rimane-in-sky
http://www.skysports.com/cycling/news/15264/10137127/team-skys-wout-poels-keen-to-lead-team-sky-at-future-grand-tour
https://www.procycling.nl/2016/10/19/bauke-mollema-2017/
http://www.cyclingnews.com/news/contador-confirms-he-will-target-the-tour-de-france-in-2017/
http://www.wielerrevue.nl/2016/07/11/kruijswijk-2017-twijfel-tour-alles-op-giro/
http://www.telegraaf.nl/telesport/26836487/__Gesink___Geen_klassement_in_Tour_2017___.html
http://www.cyclingquotes.com/news/nibali_hoping_to_target_100th_giro/
http://www.elespectador.com/deportes/ciclismo/podra-nairo-quintana-ganar-el-tour-2017-articulo-660904
http://www.cyclingquotes.com/news/pinot_in_2017_i_will_focus_on_the_giro/
http://www.nation.co.ke/sports/othersports/Froome-and-Bardet-eye-2017-Tour-de-France-mountain-duel/1951306-3421760-u3ssxlz/
http://www.cyclingnews.com/news/dan-martin-the-tour-de-france-is-where-its-at/
http://www.francetvsport.fr/videos/video-les-etapes-cles-du-tour-de-france-2017-363689