I don't think this is the beginning of World War 3. China and Russia have not indicated any interest in helping their ally Iran. Russia is preoccupied with its war with Ukraine, while China is focusing on its economic war with the US. I guess they are avoiding direct war with the US. Maybe they might assist Iran indirectly. The outcome of this attack on Iran is just unpredictable.
Russia and Iran are not allies, they are just military strategic partners and this relationship has become closer during the Syrian conflict. There is no relationship or commitment to military support between them, unlike the close alliance between the United States and Israel. Likewise, China is not Iran's ally, it is just a trading partner.
All three countries have conducted joint exercises in the Gulf of Oman to show strength to the United States and NATO, but unfortunately they do not have any binding alliance relationship. That is the main reason why they stay out of the war and do not provide military support to Iran.
Iran's allies are Hezbollah and Yemen.