Donald Trump just cracked an incest joke about having a threesome with two of his sons -- Donald Jr. and Eric -- and yes, he actually used the word "threesome" ... and it's all on video.

POTUS was at the Theodore Roosevelt Presidential Library in North Dakota on Wednesday when he started about giving out the Medal of Honor ... he spotted Don Jr. and Eric in the crowd and said ... "I see my two beautiful sons sitting there. I think I'm going to give one to myself, one to them, and we'll have a threesome, okay?"

The bizarre statement is met with applause and laughter from the audience. We can't make this up ... seriously, ya gotta watch the video.

As for why Trump would be giving his sons a medal of honor ... well, he didn't actually have a reason. He just said he'd give it to them "for something." He later said they deserved it for their "genius at hunting."

Naturally, Trump says he would give himself a medal. For what? Well how about for "taking on Russia, Russia, Russia."

The incest joke is alarming enough, but the idea Trump wants to just give himself and his sons the highest military honor in the country for these reasons is ... concerning.