Continuing with Group 3!
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Boron can be stable with only 6 electrons, does not require the full octet. And while it is hard and has low density, boron is expensive to extract so its production is low. However, Boron is very helpful in that it can be incorporated into plastics, where it creates a tough composite material--stiffer than steel but lighter than aluminum--that is used in aircrafts, missiles, and body armor.

Aluminum is one of the most abundant elements on Earth (Oxygen > Silicon > Aluminum). It was originally hard to obtain, but now it is now obtained on a large scale with the Hall Process. Aluminum is not easily oxidized because it forms a thin oxide film which prevents it from getting oxidized.  Because of its strength, lightness, and resistance to corrosion, aluminum is often used in construction and aerospace.

Gallium is used in electronics, especially in doping semiconductors.

Thorium is a poisonous heavy metal.

Carbon! Carbon is one of the most important elements of organic chemistry. Carbon also has multiple allotropes, including diamond and graphite. 

Silicon is used in semiconductors.