Hydrangea paniculatas are hardy to zone 3. Many of them get large though so pay attention to the dimensions in the plant labels. Some new entries come in smaller sizes though. Amongst the smaller ones: Little Lime, Little Quick Fire and Pee Wee. The other one maybe/probably hardy to Z3 is the H. arborescens. But that is debatable. Some websites/book/sellers
differ. Some say it is hardy to Z3 and others Z4 so be advised and touch base with your local plant nurseries to be sure. Of the various H. arborescens, Hayes Starburst usually stays around 4x4 and may do the same in your Z3 since the growing season will not be too long.