There are plenty of three speed beach cruisers out there. If you check
Chubby's website, you'll find them in your price range in both men's and women's models. Yes, the women's seem to cost more. If you're ok with swinging a leg over that higher top tube, save some money.
Some things to keep in mind: Buying online may save you some money up front, but if you can't do the final assembly, (most bikes are shipped 80-90% assembled) you'll have to pay someone to do that for you. It's not terribly difficult, especially for a beach cruiser, but you'll want to make sure the brakes work and that everything is tightened properly. Buying from a local shop, they may offer you some sort of free service for a year, (some shops offer lifetime maintenance) but maintenance on a cruiser is minimal, at best. And since they don't exactly get ridden hundreds of miles each week, not much is going to be required anyway. So don't let that be a big selling point. Same with fit - they all have roughly the same geometry, and with their giant seats, upright riding position and wide bars, comfort is rarely a problem.
I would suggest you look for something like a Micargi. They should have something to suit your budget. While $300 is really at the low end of the bike shop scale, it should still get you a decent cruiser. Do yourself a favor, and avoid WalMart bikes. Every single item WalMart sells has to be built to the lowest possible price point. And don't be afraid to check your local classified ads, such as Craigslist. Plenty of folks bought bikes with the best of intentions, and then find they don't use them as much as they thought they would. You can frequently find a like new bike at a substantial discount.