You need to follow the sign to "flight connection", will pass a line of security and passport/ticket check, before entering T3 connection desk, where Air Canada desk can take care of your next leg. They will re-issue a boarding pass, if SQ does not.

No, you do not need to go through UK immigration.

Also make sure AC to enter your luggage records, if you have any check-in luggage.

In terms of lounge, right at the entrance of departing gate pass at T3, it is AC/SAS London Lounge, which faces the entrance. That is a lounge for *Gold as well. Its location is good, in terms of just next to the duty free area, but you had better check your flight's departing time, because there is usually a long distance walk to AC flight gates, up to 10-15 minutes.

AC London lounge has four shower rooms, two floors. It is quiet in the early morning, and usually get very crowded in the late morning and after.