Couscous is a staple food of North Africa. Although it looks like a grain and is generally treated like one for cooking purposes, it is best described as a type of pasta as it is a manufactured product, made from wheat semolina.
Most couscous sold in the West is partially cooked and then dried so steaming three times is probably overkill. This type of couscous can be prepared by simply soaking it in hot water for a few minutes - although steaming it (once) will vastly improve the texture.
Ok, there is no exact rule. Sorry! I know, you probably want an exact rule and I will share my own recommendation but I would like to just explain a couple of the variants at work here.
Because couscous is a manufactured product, differences in the manufacturing process can cause some brands to cook more quickly or require varying amounts of liquid to prepare. There are tiny differences in the size of the 'grains' between manufacturers too which will impact the couscous to water ratio. Also, while perfectly cooked couscous should be light and fluffy, some people have a preference for a drier texture which again will influence the amount of water you need to add.
However, I think the best general guide when measuring couscous and water, is to use the the same volume of water as couscous. This always gives me excellent results. It also has advantage of being very easy to remember i.e. 1 cup of couscous to 1 cup of hot water. Using a metric system, the ratio is not so snappy but if you prefer to use weight rather than just volume, the equivalent is about 180g of couscous to 240mls of water.
Recipes vary widely in what they recommend - many are more generous with the water than I am - which is what causes so much confusion on how to make couscous. Personally I find being mean with the water is a good idea. It's fine to add a little more after 5 or 10 minutes if it seems a little dry.
180g/1 cup of couscous
240ml/1 cup of hot water
Salt and pepper
Steaming the couscous gives a much lighter and fluffier result. If you have time or are making a special meal, it is well worth the little bit of extra effort.