Short version: A-3 begins 3/4 mile before the Soldotna supercharger and ends at 6th Avenue in Anchorage where A-1 begins. Interstate A-1 continues to the Canadian Border. The supercharger in Chugiak is along A-1. A-1 joins briefly with Interstate A-2 along the way, between Glennallen and Gakona. Details below, and info on the roads in Canada necessary to make the trip between the superchargers in Chugiak, AK and Prince George, BC. There are two main routes of interest: sticking to the famous Alcan, and the shortest route.
It is confusing: I was driving home to Homer, AK from Anchorage, AK the other day and the car's navigator voice told me to "bear right to stay on the Seward Highway", when actually going straight would keep me on the Seward Highway and bearing right would put me on the Sterling Highway.... Up here, for better or worse, we refer to the highways by name; few residents could tell you which (Alaska) route number any particular highway is. No-one gives directions by route number...and nobody in Alaska besides me and a handful of bureaucrats knows anything about the interstate highway system numbers here. And, aside from a 40 mile stretch between Anchorage and Palmer, no-one driving on them would mistake any of these roads for an interstate highway!
Heading from Alaska to the Lower 48, Alaska Route 1 (different than Interstate A-1) begins as the Sterling Highway in Homer, AK at the junction of Pioneer Ave, and continues under a variety of names to Tok, AK. Alaska 1 becomes Interstate A-3 at the jct of the Kenai Spur Highway, 3/4 mile before the supercharger on Kleeb Loop in Soldotna, AK. The Sterling Hwy changes to the Seward Highway at an intersection about 30 miles N of Seward, but remains Alaska 1. Interstate A-3 proceeds to Anchorage where it ends at Interstate A-1 (and the Glenn Highway- still Alaska 1) at a right-hand turn at 6th Avenue. Interstate A-1 continues as the Glenn Hwy. past the last supercharger you'll see for 1852 miles, in Chugiak/Birchwood, through Palmer to Glennallen, AK where A-1 joins the Richardson Highway (Alaska 4, Interstate A-2) for a few miles to Gakona, where A-1 continues alone as the Tok Cut-off to Tok, AK where Alaska 1 ends at the Alaska Highway, which is Alaska Route 2, but still Interstate A-1, to the Canadian border. (The northern terminus of the Alaska Highway is in Fairbanks, AK)
Helpfully, in Canada Interstate A-1 continues as Yukon Territory Route 1 (YT1), where it is named the Al(aska-)Can(ada Highway), to Contact Creek, where the Alcan becomes British Columbia Route 97 (BC97) to Dawson Creek and the rest of civilization (there are actually two or three remote segments where YT1 enters BC, and then swings back into the Yukon, where I think technically YT1 changes back to BC97, then back to YT1...). Along the Alcan, it's a shorter distance to the next supercharger if you leave the Alcan and bypass Dawson Creek (mile 0 of the Alcan) by taking BC Route 29 at Ft St John, then back onto BC 97 at Chetwynd, and on to Prince George and the next supercharger after Chugiak/Birchwood. However, most travelers might be interested in the gap sticking to the famous Alcan.
HOWEVER, THE shortest route between the superchargers, which is probably of greater interest here, follows BC Route 37, the Cassiar Highway, which, heading from Alaska, leaves YT1 about 10 miles west of Watson Lake, YT (well before Contact Creek) and continues south to the junction with BC Route 16, the Yellowhead Highway, which leads east to the supercharger in Prince George, 1852 miles from the supercharger at Chugiak/Birchwood.
Most tourists drive the Alcan from milepost 0 at Dawson Creek to get to AK, because that's the thing to do, and because the Cassiar is very remote, without many services, and probably still carries a reputation for being rougher, although it was paved more than 25 years ago. It is narrower and has few lane markings. Probably the majority of Alaskans heading south also take the Alcan to either Dawson Creek, or they bypass Dawson Creek though Chetwynd to Prince George, depending on where in the Lower 48 they are headed. BC37, the Cassiar, is 89 miles shorter between superchargers, but the Alcan is only 36 minutes longer to Prince George, according to Google Maps (taking the BC29 cutoff). The route is not a straightforward interstate connection, but these are the only roads connecting Alaska to the rest of the N American Highway system. I hope this helps.
