Can not export A3 report in xls format.
There is following comment in JRXlsExporter.java class but it seems to be outdated:
// Compare to ISO 216 A-Series (A3-A5). All other ISO 216 formats
// not supported by POI Api yet.
// A3 papersize also not supported by POI Api yet.
Class HSSFPrintSetup inherits PrintSetup that has A3_PAPERSIZE so A3 should be allowed.