Ultramar Português achievement in Civ 6

Build a custom game and make sure to pick Joao III as your leader. Set Hojo Tokomune of Japan, Pedro II of Brazil, and either Gandhi or Chandragupta of India as AI leaders. Choose Island Plates or Archipelago map because Portugal can only send trade routes to other civs if they are on the coast; picking a water-heavy map type will help this. Choose a Tiny or Small map to increase chances they will spawn closer to you.

As you would do to win with Portugal anyway (

), focus on techs related to traversing the ocean. Found your first city on a coast if you can for the Sailing eureka. Then try to get (Celestial Navigation, Shipbuilding, Cartography) and trade (Currency, Banking). Send scouts and naval units out to find other civs. Each civ you find grants an extra trade route. Then build harbors/commercial hubs and buildings to get traders.

Send the traders to the foreign cities - once routes to cities for each of these 3 civs are set, the achievement will pop. Note you'll need to be at peace with the other civilizations to send a trade route.

If you're having a hard time being able to make routes - if the other civs are too far away - you can create trading posts by finishing routes to closer cities. I believe each post lets you extend another 15 tiles, and on a Tiny or Small map, one in-between route should be enough to get you there.