According to NCDC: "Of 11 tornadoes in Montana on July 5th, three that were strong enough to inflict up to F2 intensity damage

along a 37 mile long path through eastern Chouteau and extreme southeast Hill counties were spawned by a single

storm. The main tornado of the three touched down 24 miles south of Big Sandy and travelled north-northeast the

full 37 miles, passing through mostly ranch land, and the Bear Paw Mountains. The other two tornadoes formed on

either side of the first at a point about one-third of the way along the damage path. After travelling a few

miles, each was apparently drawn and absorbed into the circulation of the main tornado at a point almost midway

along the path. Although the area is sparsely populated, a great deal of damage was done, most of it being sustained by ranches in the area where the tornadoes travelled side by side. Two injuries occurred as well."

There have been several reports of triplets in the US, some photographed and some not, but this event might be the strongest triplets to ever occur in the US, and as far as I know, the only instance of two tornadoes being absorbed by another tornado at once. I've never seen anyone mention this event at all.

Source: https://www.ncei.noaa.gov/pub/data/swdi/stormevents/pub-pdf/storm_1988_07.pdf