إلى أن يبقى الثلاث وبعد ذلك لا يجوز
The process is allowed until three individuals remain; beyond that it is not allowed in this analysis.
The rule takes three as the minimum plurality required for the relevant general class. Further specification is not extended until only one or two remain, thereby eliminating the plurality represented by the expression.
This is a particular usul rule, not a claim that every general expression has the grammatical form of a plural. Words such as man and mā may convey generality in other ways.