American Rules Expert Dick Rose publishes articles about the rules in Sailing World. Usually because of questions from readers. You can have a look at his latest piece titled: What Happens When Rule 18 Doesn’t Apply?
It explains a 3 boat situation approaching a windward mark.
He also has a clear explanation about when a penalty is beginning.
The archive goes back a bit - his first article is from Oct 2001 - but still good reading. Good graphics too.
The only criticism I have is that he names the boats after a person. Persons are not protested, boats are. People on board represent the boat in a protest..... Or am I now being pedantic?