The whole of “Nearly three teaspoons of sugar” is a noun phrase and acts as subject of the sentence. Please also note three teaspoons of sugar is plural as:
one grain of wheat (singular) three grains of wheat (plural)

I once heard someone remark "Australia has no real culture", in this context I took it to mean they lacked a history of "high Culture", or perhaps they were putting down enthnic values/lifestyle of Australians. In this case I guess there might be an ambigious overlap.