比较合适的翻译应该是I am about to graduate.
解释：
be about to do sth. 即将做某事，通常指在极短时间后将要做的事。
例：I am about to leave. 我马上就要走了。
be going to do sth. 将要打算做某事，通常指有一定计划性的决定。
例：I am going to marry with James. 我打算和吉姆斯结婚了。
will do sth. 未来会做的某事，通常指临时起意的某个决定。
例：I will help you to move this table. 我来帮你搬这个桌子。
试区别：
A: I am about to marry with James.
B: What? Are you kidding me? You don't even ware wedding clothes.
A: I am going to marry with James.
B: Really? When and where are you going to do so? I would like to attend your wedding.
A: I will marry with James.
B: Well, he is a handsome guy. Good luck to you...if he "love" you.
因此：在“我即将要毕业”句子中
I am going to graduate. 表示我计划要毕业。通常应该为 I am going to graduate in three years. 表示有强烈计划性质的毕业，可能是因为提前读完，或者转学等。
I will graduate in next year. 表示我应该是来年毕业，暗含一种我想，如果不出意外，应该是的意思。
I am about to graduate. 表示我马上就要毕业了。可能后面会加上一两句话，比如，就差个毕业考试了。或者就差拿到毕业证书了。
综上，在本提问中的句子，选择I am about to graduate.是最为合适的。
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