Hi Valcav,

You can easily walk from T3 to T1, but the biggest factor will be the Customs hall in T3 when you land. On my recent trip, I also had a 2hr connection between flights and was transiting T3 to T1 - I was also on AC864 but I was in J and was fortunate to have the pink "fast track" slip that sped me through Customs, but the line in the hall that morning was EXTREMELY LONG (at least > 250 people in line), and not that many kiosks open, so just factor that in. But I also had an extra 20 minutes than you do (630 landing, 840 departure at T1), but I also sat in the BMI lounge for about 30 minutes, in addition to showering at the Arrivals lounge before heading over. If there is a medium to large line at Customs, you most likely will not have time to go to the Arrivals Lounge, unless you clear Customs and Baggage (do you have carry-on or checked) within 15 minutes of landing.

The walk should take you approximately 15 minutes from T3 to T1 through the tunnel - its very easy (especially if it doesn't take you 10 minutes to find the tunnel like it took me, since I was a bit scatter-brained that day, ha ha).

Good luck and safe travels!