Step 1: See a dentist, have cavities treated asap. Yes, these teeth will fall out, but until then, they act as placeholders for the permanent teeth. There are also sources that claim the deciduous teeth are important for the development of the permanent teeth. (Which will start to appear at around five to seven, that's quite different between kids.)
And frankly, untreated your daughter is in for a lot of pain if you don't act now. If the teeth are in really bad shape, a good dentist might suggest sedating her to treat the cavities. While this sounds extreme, it may be a good choice, because conciously having multiple teeth treated is no picknick for a 3.5 year old. Ask around for recommendations, you want a dentist that is good, patient and gentle with children and takes his time, not just one that does a good job with the teeth alone.
Step 2: Start brushing teeth as recommended: At least twice a day using toothpaste with appropriate fluoride content and as per recommended technique. Your dentist or the Internet can guide you. Brushing teeth is the parent's responsibility until the child's dexterity is quite good. Until then, let the child brush first to practise, then finish. Rule of thumb: by the age of seven or eight, they should be ready. A 3.5 year old can't manage this.
Step 3: Visit the dentist regularly, typically twice a year. This not only prevents fear in the child, but ensures small starting cavities can be treated before they become problematic. A good dentist will also help you with proper technique, because he sees the spots you might be missing when brushing her teeth.
Going to the dentist only if you are in pain might establish a bad pattern for life.