AMCo. is a oil company from Grand Theft Auto III, and Grand Theft Auto: Liberty City Stories. There is a small branch in Trenton on Portland but it hasn't any big function related to the size. You can find it opposite to Joey's garage. In GTA III you can find behind a fence on the southeast side - on the grounds of the Burkes adjacent building - armor. There is a Hidden Package hidden on a green space on the southwest side.
Furthermore AMCo. has a gas station in Harwood, in the North of Portland, but you can't fill up your car. A Hidden Package is in GTA III located on the roof of the store, and next to the north entrance you can find a heart item. In LCS, have a look at the roofage of the gas pumps, there's a Hidden Package, too.
In the district Torrington on Staunton Island there are the AMCo. headquarters which are very huge. Underneath the building there's a underground parking lot, and on the southwest side there are stairs you can use to reach the roof garden (both locations have a Hidden Package).