Enter those item IDs in the field with SEMI-COLON as separator.
Typically ; is used as a separator, although you can change that using preferences.
What do you mean by "Open"? When you Open an Item in Teamcenter, Teamcenter will create new tab in My Teamcenter application (left panel) and display the Item/Revision information there.
I am able to select 3 items and Open all of them in their respective tabs.