..is a message I've got in my head, from somewhere.
True, False, somewhere in between?
Ignore the message re: no carbs after 3pm...
The portion and type of carbs you consume, is more important than when you consume them. Approximately 50% of your daily energy intake should be from carbohydrates (they are also an important source of fibre and help with concentration/brain and kidney function).
By type - avoid refined carbohydrates (e.g. white rice/bread) and opt for complex carbohydrates with a lower glycemic index (e.g. wholemeal breads/pastas or brown rice).
As for portion size - this is very much dependent on your output/how much exercise you do. A lesser known fact is that too little carbohydrate can be really counter-productive - if you deprive your body of carbs, your muscles will try breaking down the proteins you eat until they begin acting like carbohydrates...
A good RJ article that discusses this stuff a bit lives here: http://www.realjock.com/article/94/