想在欧洲第三国签canada的transit visa经加拿大回美国
看Canada visa网站的要求
Applicants for Temporary Resident Visas, Study Permits and Work Permits
As of April 1, 2005, you must apply in your country of residence, your
country of nationality or the country where you have been legally admitted.
此处就是讲合法入境吧 只要护照上有入境的戳就行吧
多谢