what is this on my 3 month old?

Hard to say for sure just from a description alone, since skin things on babies can range from something really harmless like a heat rash or milk spots to something that needs a closer look. At 3 months the skin is still so sensitive and new, so spots can appear for so many different reasons. It might be worth bringing her to the PD just to be safe, since they can tell you straightaway what it is and whether anything needs to be done. Better to know for sure than to guess lah.