This doesn't just apply to 3CP, but unifying all of your routes together after each point is a good idea. Also, once red loses a point they should be pushed back, with combat not happening right after the point.
Another thing you'll probably want to keep in mind - it's nice to have a difficulty curve of sorts between the three points, with A being the easiest, B being harder and C being the hardest. It seems obvious when you think about it, but I keep seeing 3CP maps in which B is either an awful choke (and you never see C, and it feels like you should just cut out C and make B the new last), or B is lost immediately to blu because red can't establish a hold (and again, it feels barely worth keeping it as 3CP over 2CP). In my opinion, getting a good balance between red being able to hold B and blu being able to capture in a certain timerame is one of the harder bits to balance when you're making a 3CP A/D map, and it might be worth just making it as 2CP A/D if you don't want to worry about that (but once you've got that balance it's super neat, and I personally prefer a good 3CP over a good 2CP because of the B point)