What are 3 different things that Igor Gouzenko did that were of importance?

I really don't think there are three separate things that Gouzenko did that were of importance.  To me, he did one thing that was of importance -- he defected.  But his defection had a lot of important consequences.  So I would say he did one important thing that had many consequences.

First, his defection can be said to have helped start the Cold War.  This is because it caused there to be more suspicion between the Soviet Union and the West.

Second, the information he brought helped reveal Soviet spies in three different countries.

Third, his revelations made Canada start to be more conscious of spies and the need for counterintelligence.

So his one action led to at least three major consequences, but it was just one action on his part.