So, looking through some of the Buddha biographical collections in Chinese, I did find the three similes in a couple other parallels.
The earliest of the biographies translated during the Latter Han dynasty is T184 (~150 CE). It has a much briefer version of the Bodhisattvaās early practice. It lacks most of what we would expect from the story like the laundry list of extreme practices and such. It doesnāt have the three similes or much of narrative about the Bodhisattva traveling from one place to another. Instead, we are simply told that he ate very little and the gods fed him to keep him alive during that time.
T185 was translated during the Wu dynasty (~250 CE). It reads very similar to T184. The six years of practice take up only a couple lines of a text about his eating only a grain of sesame or rice each day. Thereās no mention of the three similes.
Those two early translations were also quite brief and generally only covered the story of the Buddha leaving home and defeating Mara.
T186 ę®ęē¶ was the first large collection of stories. It was translated ~300 CE. The Bodhisattvaās six years of austerities is chapter 15. Itās a much more involved story, but it doesnāt include the three similes that I can see, before or after the six years.
In T187 ę¹å»£å¤§čå“ē¶, which I believe is the MahÄvyÅ«ha. It was translated during the Tang dynasty, around 683 CE. The Bodhisattva contemplates the three similes after abandoning his initial teachers and going off to stay on Mount Gaya next to the NairaƱjana River. The passage occurs at T187.3.580b21-c14. He then leaves the mountain and goes to a lake in Uruvilva where he could look at the NairaƱjana River. Admiring how beautiful it was there, he finds a place where he begins his six years of practice. So, that agrees with the Pali suttas.
T188 doesnāt seem to describe the austerities, being an excerpt of a story collection.
T189 is also a larger collection of stories. It was translated in 435ā443 CE. It includes a narrative of how the Bodhisattva practiced for six years eating very little, but most of the story between his deciding to do this and decided to stop is about King Bimbasara and his family being unhappy with the situation, since he appeared to be course to starve himself to death. Thereās no mention of the three similes.
In T190 ä½ę¬č”éē¶ was the largest collection of stories translated to Chinese, comparable to the Mahavastu. It was translated during the late 500s CE. Chapter 29 is the story of the Bodhisattvaās austerities. The Bodhisattva goes to stay on Gayasirsa Mountain and there he contemplates the three similes at the outset of his six years of austerities. After contemplating the similes, he comes down from the mountain and finds a place to practice at Uruvilva. So, that also agrees with the Pali suttas.
So, I have to say, the evidence doesnāt make me very confident that this story element is very old, given that the earlier versions lack this bit of the story. The Mahavastu and Mulasarvastivada Vinayas are not particular early sources, either. Maybe the reason it seems off in the Pali suttas is because it was inserted later in an awkward way to mimic these other story collections that included it?