In Apex Legends, there is no exact time in which the final stage of the match begins. You can assume that it begins in a moment when there are only a few teams left on the Arena, while the enclosing ring forces them to fight in a very limited area. Sometimes, the whole match can end in round 3, but sometimes it can last longer, up to the fourth round.
Round 3 can sometimes be the final stage of a match, so you need to prepare for it thoroughly. The round lasts 3 minutes, while the combat takes place in a very limited space. The remaining squads do not come out of hiding, waiting for a move from the enemy, and canvass the area using sniper rifles.
The ring - third stage
At this moment, the ring encloses really fast, and the players have much less area to walk around freely. If you need to move to a safe area as fast as possible, look around for ziplines or red balloons that will facilitate this task.
Round 4 is typically the final stage of a match. The teams that managed to reach this stage are armed to the teeth - equipped with weapons, an armor of the highest level, and a lot of ammunition and medkits.
Find a vantage point
The best way of winning a match is finding a good vantage point from which your team will have eyes on the whole circle. Using weapons with a good zoom will enable you to canvass the area searching for a potential target.