Does the latest tourism law stipulate how much cash to carry when traveling to Singapore, Malaysia and Thailand?

Thailand has. If you go to Thailand and landing visa, you must bring enough 2w baht in cash or equivalent RMB, otherwise you will be sent back to China if you are spot-checked. There is no requirement elsewhere, but it is not very convenient to swipe your card in other Southeast Asian countries except Singapore. You can't swipe your card for some daily consumption, so you'd better bring a certain amount of cash.