「ちゃん」is used when addressing someone who you are fond of or close to however,
in many cases it is used towards smaller or cuter people or even pets.
「さん」is more commonly used when showing respect to someone so it is very widely used.
「君（くん）」is used towards a male who one is close with but it can be considered respectful in some situations as well.
I hope that clears up your question. all 3 of these can be used on pets as well.