三选一用英语怎么表达

我觉得还是不合适用either,or
你这话让我理解成,要么选旁散激择A,B,要么C; 与你想要表达的不一致.
可运袜以这掘销样改吧:
Then they were randomly assigned to three conditions,in which they can choose one.

Select one out of three.

Then they were randomly assigned three conditions, in which they were supposed to select one out of the three.