Conservation of Time
After Crono reaches the End of Time, Gaspar introduces a Conservation of Time Theorem, noting that only three people can pass through the Entity Gates.
When 4 or more beings step into a time warp, the Conservation of Time theorem states that they will turn up... ...at the space-time coordinates of least resistance. Here. [End of Time]
The Japanese version of the game elaborates that at least one of the four beings must be from a different time period than the others, though this is still completely arbitrary and used to reason why the party must have three members at all times.
The veracity of the Conservation of Time Theorem is debatable, as it probably stemmed from a need to limit the party to three characters at any one time to conform to the game's RPG system. The Conservation of Time Theorem is also violated a few times; specifically, when Magus summons Lavos at his castle, he ended up in 12000 B.C. while Crono and two others went to prehistory. Secondly, in the original Ocean Palace disaster, two Gurus and Janus went to another era normally, leaving only Gaspar to arrive at the End of Time. The biggest infraction on the theorem is the Time Crash, which sent the entire Sea of Eden, housing countless researchers and workers at Chronopolis, ten thousand years into the past. Notwithstanding these contradictory instances, a defense can be raised. These three problem instances differ from normal Gates in that they were caused by temporary distortions. The Conservation of Time theorem thus may only apply to the blue, permanent Gates made by the Entity; black Gates and other distortions (even if Entity-orchestrated, like the probable Ocean Palace Incident), would not adhere to this rule.