Unassisted double plays are uncommon, but they happen. First or third baseman catches a low line drive and tags the runner who led off and/or took off thinking there was no way you'd make that catch. Second baseman catches a one-hopper between first and second near the baseline, tags the base runner leaving first and beats the batter. Shortstop does the same with the second and first-base runners.
An unassisted triple play, however, seems impossible without a huge blunder on the part of multiple runners or the umpire, especially with the IFR. The only way I can think of that it would realistically happen without a major runner blunder is for the shortstop, positioned close to the baseline and near second, to catch a screaming one-hopper, tag the runner leaving second, tag second base forcing out the first-base runner, and then it's a footrace to first and the batter has a head start. The batter would have to be trotting to first for some reason (injured, older slugger).
The only other scenario I can think of is similar, but involves a pop fly and a sleeping umpire. Batter pops the ball high and right to second base, umpire doesn't signal IFR but batter's sure he's out. Shortstop or 2B muffs the catch, picks the ball up, tags the second-base runner, tags second base, and races the batter to first.
Wikipedia's possible scenario has the second baseman or shortstop standing between the second-base runner and second base itself, and the second-base runner would take pains to prevent that, because if the catcher saw that he'd signal the pitcher for an easy 2B pickoff. It also requires the first-base runner to not see the catch before committing to take second base. Either way, Wikipedia's scenario requires multiple baserunner mistakes on top of a perfectly-placed hit and baseman.
Are there any other scenarios likely enough to be plausible in MLB?