Initially, Zhang Sanfeng was a little monk in the Shaolin temple. He was there for around 15 years, so Buddhism tenet must have had a strong influence on him. Futhermore, he learned 9 Yang Zhen Jing. Though 9 Yang was a mix of the 3 Religions in the Ancient China, it was still a book of Buddhism scriptures.
That makes me not able to understand why Zhang Sanfeng became a Taoist instead of returning to the secular life or moving to another temple to keep training as a monk after he left the Shaolin temple.