C.R.
Nearly all of the malls in the cities have indoor play areas (Maplewood has a really nice one). There's also an indoor playground at Northdale Recreation Center (1414 Albans St, St. Paul) that's toddler appropriate. There's also Edinborough in Edina, Adventure Park in Ham Lake, and indoor play areas at most of the community centers throughout the cities. Some of the community centers have indoor pool areas, but I'm not sure I'd want to try that with three small children. The malls and Northdale are probably your cheapest options as they're all free. The others are usually about $5 per child.