US Airways arrives at Terminal 3 in Toronto, and Alitalia departs from the Infield Terminal.

When your plane lands at T3, you will need to clear Canadian customs and retrieve your bags temporarily. This is so that they are on your person when you pass by the Canadian inspectors. 9 times out of 10 they don't even look at them, but them's the rules. If your bags are tagged through to your Alitalia flight, you can drop them at the re-check belt in T3 and then proceed upstairs to your bus to T1, which you catch on the roadway in front of T3 (bus stop signs hang from the roof of the building). You can't walk between the terminals. The buses run every 8 minutes.

You need to go to T1 because all the check-in facilities for the Infield Terminal are there. The Infield Terminal is a temporary building with gates only that is in use until T1 is finished. Passengers and bags are taken by vehicles (in your case another bus) out to the Infield once they have checked in at T1. If US printed your AZ boarding passes, you can go straight to security at T1. If not, stop by the check-in desk first (you will need to do this anyway if US didn't tag your bags thru).

Security in Canada is marginally quicker than in the US... friendlier too. Lots of security lanes open at peak times. 10 minutes is long for me in Toronto.

Total time needed on the ground in Toronto is about 90 mins to 2 hours depending on how fast everyone in your party can walk. The main variables are going to be your wait at Canadian customs at T3 and the vagaries of the GTAA bus schedules. 3 hours should not be cause for alarm.

More info:
Greater Toronto Airports Authority
www.gtaa.com

Feel free to ask me any additional questions.