We just use a big, shallow rubbermade-type tote (the under-bed storage style; I bought ours at Big-Lots). The lid stays on really well and of course keeps out critters and rain. It's great because at the end of the summer I rinse it out well and then use it to store outdoor toys during the winter. It may not be big enough for your needs, but it's worked great for us! 2 kids can easily play with ours at the same time...you might need a slightly larger one for 3. (I also like it because it's big enough for the kids to play with, but not so big that they actually walk around inside of it getting sand all in their socks.) Super-cheap!
Mom to J 6/25/08 and M 6/21/11