Sun wrote his version of the Three People’s Principles around 1906. The Bolsheviks’ October Revolution didn’t happen until 1917.
I think you can say Sun was a lefty but not a Marxist when he first came up with the Three People’s Principles. Although he really only came into contact with communism until 1922, when his fell out with his warlord buddy in Guangdong and found himself without an army.
By then the Soviets had won WWI, and was willing to fund and back Sun’s KMT as the main branch of the communist party in China. Since the Soviets chose the KMT as the representative of Communist International in China, the entire CCP joined the KMT in 1922. As a result Sun was really impressed by the Soviets and Marxist philosophy.
So much so that in 1924’s first National Assembly, Sun mentioned in his speech that the Minsheng part of his Three People’s Principles, a.k.a. the People’s livelihood, is the same as socialism, and also can be called communism, which is the realization of Confucian’s the Great Unity.
民生主義就是社會主義，又名共產主義，即是大同主義。
So don’t tell me he wasn’t Marxist by 1924. It’s hard to tell whether or not he was a true believer of Marxism, or if he did and said stuff just so that the Soviets would keep their aids coming, but at least he remained pro-Soviet until he died.