Tier 3 was designed to facilitate work visas for lower-skilled and unskilled workers from outside the European Economic Area (EEA) and Switzerland. However, the United Kingdom has decided to reserve their low-skilled and unskilled labour market for citizens of the EEA and Switzerland.
If Tier 3 is ever implemented (and there is much pressure to do so from employers in the UK), then it would most likely cover work in sectors such as agriculture, hospitality, and food processing.
It is unclear how the UK would implement the points based system for this tier.