Explain the value of population, health, and environment to global development.

Population, health, and environment are all elements that can make a positive contribution to global development or impede it. Economists have studied these three factors extensively.
Population: Total population has a positive effect of translating into a larger workforce, but overpopulation can contribute to famine, water shortages, and other negative environmental effects that can impede development. One of the most important demographic factors in development is the dependency ratio, namely the ratio of dependents (young children and the frail elderly) to working-age population. One of the major boosts to development is a "demographic dividend" or "demographic transition" as improved food supplies and medical care, often provided by outside donors, lead to a rapid increase in working-age population, often accompanied by cultural shifts which move women into the workforce. This period is normally fairly short, as it is also one in which birth rates typically decline, as parents have fewer children due to decreased infant mortality reducing the need for a high birth rate and an economic premium for having a smaller number of well-educated children. China is an example of a country that used its demographic transition to become a manufacturing powerhouse.
Health: Health issues, and especially epidemic diseases such as HIV and malaria, can impede development by increasing the dependency ratio and decreasing productivity. Poor health or nutrition in infancy can lead to developmental problems and stunting, making workers less productive when they reach adulthood.
Environment: Environmental issues can affect public health. Also, factors such as overpopulation can lead to a decline in water and food quality and availability. Environmental degradation can reduce agricultural productivity, leading to food insufficiency. Global climate change can also increase the spread of epidemic diseases, devastate coastal community, and lead to water shortages.
https://www.unfpa.org/demographic-dividend