Am I wrong to say that the highest goal in buddhism is not buddhahood but arahantship?
If by Buddhism, you mean all of Buddhism as it currently exists, then of course, Mahayana disagrees with this view.
Furthermore, traditional Theravada also accepts the three vehicles, as Buddhaghosa says that there are three main soteriological paths: the path of the Buddhas (buddhayāna); the way of the individual Buddhas (paccekabuddhayāna); and the way of the disciples (sāvakayāna). This idea was also accepted by almost all the early Buddhist schools, Sarvastivada, Mahasamghika, etc.
So in Theravada, the highest goal is still Buddhahood as far as I know, and the bodhisattva’s journey to Buddhahood is a big part of the Jataka and vamsa storytelling tradition, which a beautiful and valuable part of Theravada, even if it’s not EBT.
So it seems the three vehicles idea is pretty old and goes back quite far, since it was shared by most of the major Buddhist schools.
I am not quite sure what the earliest text we have is that discusses the idea however, but you’re right that its not an EBT idea. However, its definitely Buddhist. In fact, its the majority view among the Buddhist schools throughout history.
Interestingly enough, later Mahayana collapsed the three vehicles into one vehicle, ekayana (all paths lead to the same goal, buddhahood in this case, as explained most famously in the Lotus Sutra, the king of Mahayana sutras), and this is pretty much the standard view now in all of Mahayana. So basically, now Mahayana accepts that all Buddhist paths lead to one goal (which is closer to early Buddhism in my book), while scholastic / Abhidhamma Theravada does not (since they still accept Buddhaghosa’s philosophy as orthodox).