Is it really necessary to undergo expensive dental work on a 3 year old child's baby teeth?

Possibly: If a three year old has rampant decay throughout the mouth, extensive and likely expensive dental treatment may be indicated. Even though most baby teeth will be replaced by permanent teeth, premature loss of baby teeth can contribute to problems with the eruption and development of the permanent teeth. In addition, untreated decay can lead to more serious problems.

Establish Goals: Lets take out the word "expensive" from your question. Either you accept or reject the importance of oral health. Not treating dental disease can lead to serious medical conditions. A single cavity if left untreated can create spacing and orthodontic issues. Even more significant is that an untreated cavity can lead to an abscess or facial swelling or worse the travel to the heart and brain.

Economics: I understand that dental treatment can be expensive, but treating a child's problems is much less expensive than treating an adult's problems. The most common dental problem in children is decay (infection) that can permanently harm the underlying permanent teeth, jaws, gums, and spread through the circulatory system. To not treat infection in a child is child neglect.

Depends on type: I would not invest in expensive "normal" looking verniers or enameled caps in small kids.That said, failure to provide basic dental restoration can lead to a variety of serious health issues, impair adult spacing and impair dental health for the rest of their life.