What are three major contributions made by chemists that greatly helped humanity?

Here are three examples of the application of chemistry
for the benefit of worldwide humanity.

1.  Food
Preservatives Chemistry:  Since the time of cavemen there has been a need to preserve
food so that it can be safely stored and eaten later, preventing wastage.  In modern
times with worldwide commerce there is a need to preserve fresh foods and produce, and
to permit safe transport of these goods to regions in which the food items are not
naturally available.  In modern times food preservation technology makes food safe and
nutritious.  It permits packaging and labeling for the consumer, telling her of the
safety of the food, its nutritious value, and a safe expiration date.  Because of this
technology modern man is no longer subject to the health hazards posed by contaminated
or decayed food products.

2.  Pharmaceuticals:  Chemistry,
particularly synthetic organic chemistry, along with pharmacology and fields in biology
allow the pharmaceutical industry to produce medicines that are safe and effective. 
Through use of modern pharmaceuticals, physicians are able to treat and cure diseases
including infectious diseases that in earlier days were fatal.  The pharmaceutical
industry as well as food preservative technology can share much of the credit for man's
increased life expectancy over time.

3.  Environmental
Chemistry:  This is the study of chemical and biochemical phenomena that occur in
nature.  It is through environmental chemistry that we now have an appreciation of some
of the causes of (and possibly preventions of) global warming.  In modern times, the
application of environmental chemistry has effected a vast reduction in environmental
pollution.  In the past two decades, one of the best examples of this reduction is a
several-fold decrease in air pollution from
automobiles.

Explanation of
references:

The first reference is a tutorial on food
preservation chemistry. It includes the historic development of preservation
technology.

The second reference defines pharmaceutical
(medicinal) chemistry.

The last reference is a tutorial on
global warming, including detailed information about the chemistry
involved.