Triangulation of iPhone bearer within oitdoor space

I’m interested in whether it is yet possible to track the course of a (device bearing) individual within a 30m x 60m outdoor space using three beacons to triangulate. It would be optimal to be able to track their position as it constantly changes whilst remaining precise.
Do such beacons exist and can this be done?
If so how can I get one?
Many thanks Helen

Just three beacons, and just triangulation won’t give you very good accuracy. What level of accuracy would you need for your use case?

If this is outdoor, why not use GPS?