To guess a tripoint, type the first 3 letters of the three countries in any order. These are the first 3 letters of the standard JetPunk name, so for example you could type AUS-FRA-GER, FRA-GER-AUS etc for a hypthetical Austria-France-Germany tripoint. For some countries you are also allowed 3 different letters (eg UAE, UK, DRC...).
Tripoints data from Wikipedia, adapted to JetPunk's 196 countries