I do water aerobics and have a Series 3 Apple Watch. Can I use it while doing water aerobics?

The Apple Watch series 3 is water resistant, but beware, this means NOT that the watch is waterproof

Apple Watch Series 2 and newer can be used for shallow water activities like swimming in a pool or ocean. However, Apple Watch Series 2 and newer shouldn't be used for scuba diving, water skiing, or other activities involving high-velocity water or submersion below shallow depth. 

Water resistance isn't a permanent condition and can diminish over time. Apple Watch can't be rechecked or resealed for water resistance. The following can affect the water resistance of your Apple Watch and should be avoided:

• Dropping Apple Watch or subjecting it to other impacts
• Exposing Apple Watch to soap or soapy water (for example, while showering or bathing)
• Exposing Apple Watch to perfume, solvents, detergent, acids or acidic foods, insect repellent, lotions, sunscreen, oil, or hair dye
• Exposing Apple Watch to high-velocity water (for example, while water skiing)
• Wearing Apple Watch in a sauna or steam room

Yes,see here;

Swim with your Watch