Our understanding is that those in Tier 3 (as well as everyone else) should aim to limit travel around the UK as much as possible, especially from more severe tiers into lower-level tiers. In Tier 3, "you can continue to travel to venues or amenities which are open, but should aim to reduce the number of journeys you make where possible."

In Tier 3, you should also "avoid travelling to other parts of the UK, including for overnight stays other than where necessary, such as for work, education, youth services, to receive medical treatment, or because of caring responsibilities. You can travel through other areas as part of a longer journey."

That's from this source: www.gov.uk/guidance/local-restriction-tiers-what-y...n