Nothing says "You Can't Sit With Us" like Donald Trump's 3 predecessors crashing the Presidents Cup to watch the first tee shots -- sans the current Prez ... who loves golf. 

Barack Obama, George W. Bush and Bill Clinton attended the trophy presentation together Thursday at Liberty National Golf Club in New Jersey, before the foursome matches kicked off between America and its international competition.

The ex-Presidents looked super chummy, greeting the crowd and shaking hands. The irony, of course, is Prez Trump is a big-time golfer, and even has a golf course just 40 miles away from where the ex-Presidents were chilling.

Then again, Trump has bigger things to worry about now -- while these guys have done their duty already.