I think that  depends on the dogs character, the socialising of the breeder.

My first co pup didnt liked people from the start. Didnt see very much by the breeder and was suspicious of everything and everyone.

My second co pup was socialised by the breeder, had an open attitude and researched what was new. Friendly to everyone until he reached the age of one year old, the open attitude disappeared and the guarding inctinct kicked in. That was a huge change.

they were very diffrent , they had a very diffrent way of dealing with things but i think the outcome was the same. 

One thing will always be a trigger and that is agressive behaviour of people. 

I think with three years they are mature.