Three grams of mushrooms is easily 4 to 10 individual mushrooms. Usually people simply weight the entire mushroom and not the exact psilocybin content. This is not ideal since potency will vary but it's enough to give you a good idea of the dose when comparing to previous sessions. However, there are more and more legal services who can provide already weighted product with the exact psilocybin content on the label.

Probably just means 3g of actual mushroom fungi matter. Just need a food scale.

A kitchen scale works well for masses in this range.