Tables are always displayed from left to right, from top to bottom, so you need to add A, then C, and then finally B.
PdfPTable table = new PdfPTable(2);
table.addCell("A");
PdfPCell cell = new PdfPCell(new Phrase("C"));
cell.setRowspan(2);
table.addCell(cell);
table.addCell("B");
iText , . - , . A R1C1, B R1C2, , , , .