Is this baby acne, milk rash or heat rash? Few pics attached. Do I leave it to heal or apply something on it?

Sorry mummy as much as I want to help you I am unsure of what rash is it. The rash appeared infected from the pic. I would bring baby to see the doctor: GP or PD asap. Meanwhile, clean the rash with cooled boiled water with cotton wool. Do not re-use the cotton wool after one swap. Keep it airy. I would not apply any topical cream till I am clear of what the rash is.