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父母有绿卡但是在回中国前没有办理旅行证，他们已经离开美国快3年了，请问他们怎样才能回美国。

-Technically, their green cards are void. My suggestion: they had better abandon their green card and then, apply for B2 visa to enter US. Then, you can sponsor their green card again as you did for them before if they want to live in US permanently.

Here is the link for green card abandonment in China:

Abandonment of Status (Form I-407) - U.S. Embassy & Consulates in China (usembassy-china.org.cn)