I do not know what they ask the three year olds, but they specifically instruct parents "do not coach" your child. They say they can tell quite easily when children have been coached and they are suspect of those kids' language skills when they answer a question wrong (i.e. If they ask favorite color and the kid says I am three years old). They said they do not want a child who is behind in English because it is not fair to that child. They also say don't dress them up either. They said one 5 year old showed up in heels one year and could barely walk Just have them wear something comfortable.
Our son is shy and a bilingual. When he didn't answer questions because he was scared they wrote in the assessment does not speak English. Later when they got to the math portion, in which he excelled, they had to reassess their own views on his English skills as they were too quick to judge (he rarely speaks Mandarin now).
You know that GSIS is one of the most sought after and competitive places in HK? I mention because I really think you should take some time and ring a few other schools while here, such as Hong Kong International, ISF Academy, Canadian, Singapore, Chinese or French International Schools as well. You also might not like the facilities when you see them next to what looks like a squatter village and a major road.