I'm certainly not speaking as an English teacher here. I wasn't able to find any three-preposition (or even three-particle) phrasal verbs in a quick search. One possible example that comes to mind is "come on in around," such as in "Come on in around the back." This simply means: "Come in the back." Other adverbs/prepositions (e.g., above, below, outside) could also work in similar constructions. The control tower could tell a pilot, "Come on in below 1000 m in altitude when you reach..." He might even say, "Come on in AT below 1000 m...."!
Some of the particles in the examples may be acting as adverbs rather than prepositions. I'd be interested in any comments a teacher, grammarian or other English expert might have on any part of this.
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