by twos and threes

1. 三三两两
   The students are coming into the classroom by twos and threes.

   学生们正三三两两地走进教室。

• put two and two together
  draw an obvious conclusion from what is known or evident.
• that makes two of us
  informal that is true of me also.
• two by two (or two and two)
  side by side in pairs.
• two can play at that game
  informal used to assert that one is equally capable of copying another's strategy, to their disadvantage.
• two cents (or two cents' worth)
  N. Amer. informal an unsolicited opinion.
• two's company, three's a crowd
  two people, especially lovers, should be left alone together.