The unabridged Oxford Dictionary is the English language bible equivalent. Go to the Oxford Dictionary for exacting knowledge.
The following are my understandings of these words - usage, utilization, employment (it is not gospel, but a simple guide) which I have gleaned over 36 years working for the Department of Defense:
Use is simple and works in most all cases and is universally understood at all reading levels.
Utilize is shunned in most literary circles because it is one of the most misused words in the English language and it is frequently used to make the author seem more knowledgeable. Utilize, means to use something in a way other than originally intended as in new ways or it can mean to begin reuse of an item left idle or thought obsolete.
Employ is more dynamic. It can mean to hire or have a contract for a person(s) work or services. It can mean to actively engage an item. Frequently, employ is used with the logistic transport of items to a location that they will be used or of best use as with weapons, drugs, chemicals, medicines or transport systems.