our gpo's are rated for 10A, but any good sparky would run wire rated to 13A, then again, they are hard to find today, heck its hard to even find ones with a valid or non expired one these days,
I have seen some of the dodgier ones running 7.5A rated for GPO's, so yeah, if you need 3KW, use a 15A gpo, worst case they run 13A rated wire, and that still covers 3KW