In the core game series, Tyrogue evolves into one of its’ three forms based on it’s highest stat. The same behaviour has been confirmed in Pokémon GO.
According to a research done by DrWolfenstien of TheSilphRoad fame, Tyrogue’s evolution is entirely dependent on its highest stat.
Any ties in stats will be a random choosing of the tied outcomes. That being said, ties between Attack/Stamina or Defense/Stamina tend to lean towards Lee and Chan respectively.
A combination of all three is a little more evenly distributed. You can figure out the highest stat by using the in game Appraisal.