Reply

父母有綠卡但是在回中國前沒有辦理旅行證，他們已經離開美國快3年了，請問他們怎樣才能回美國。

-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)