[morzh]

There are two types of cans with more than one insulated terminal.

One is where one is plus and another is minus, and the can is unconnected.
Another is (and this is probably what you have) where the can is minus and all the insulated terminals are plusses, so there is more than one capacitor inside with all the minuses combined and connected to the can.

You can see how it was secured to the chassis: if it is with a clamp that hugs the can withouut any insulation and connects directly to the chassis, the can is the chassis then.
If the clamp has an insulation between itself and the can, usually a piece of dense insulating paper, then the wire connected to the can probably goes elsewhere and not to the chassis.

Often times that wire goes to a large wire-wound bias resistor that creates some negative bias low voltages between the most negative point of the schematic (the neg. output of the rectifier, like sometimes it is the centertap of the respective winding of the power tranny) and the chassis. This is one case where the chassis connects to a capacitor's plus and not minus as in most cases.