According to the International Swimming Federation (FINA), a child of 3 years old is not capable of floating, as they still lack the necessary upper body strength and coordination to support their own weight in water.
Typically, it’s recommended that children start learning how to float around age 4 or 5, when they have developed sufficient physical abilities. By this stage, most kids are able to develop a basic understanding of buoyancy and can slowly learn how to position themselves correctly on the surface of the water.
In contrast, for an adult with a height of about 100cm (3 feet 7 inches), floating would likely be possible, as they have developed more significant upper body strength to support their weight in water. However, it’s always important to note that flotation capabilities can vary greatly depending on factors such as swimming ability, physical fitness level, and the specific environmental conditions.
In this particular case, for a 3-year-old child with a height of about 100cm (not accounting for any developmental or growth-related effects), I would still recommend waiting until they are slightly older before introducing them to water activities that require floating.