I was hoping to wear my Apple Watch 3 on the beach. Is it protected from sand?

Apple Watch Series 2/3 are water resistant, and sand is rather larger than water molecules so sand is unlikely to be able to penetrate the seals. About Apple Watch water resistance - Apple Support

However be careful if sand gathers and gets trapped under the digital crown, it may block the action or get ground into the watch. Abrasion could also be a risk depending on the beach material and your activities.