//Load three different PDF files
PdfDocument doc1 = new PdfDocument(pdfFileName1);        
PdfDocument doc2 = new PdfDocument(pdfFileName2);        
PdfDocument doc3 = new PdfDocument(pdfFileName3);        

//Merge them into a single PdfDocument
PdfDocument mergedDoc = PdfDocument.Merge(doc1, doc2, doc3);

//mergedDoc is a new PdfDocument object containing all
//pages from doc1, doc2 and doc3
....

'Load three different PDF files
Dim doc1 As New PdfDocument(pdfFileName1)
Dim doc2 As New PdfDocument(pdfFileName2)
Dim doc3 As New PdfDocument(pdfFileName3)

'Merge them into a single PdfDocument
Dim mergedDoc As PdfDocument = PdfDocument.Merge(doc1, doc2, doc3)

'mergedDoc is a new PdfDocument object containing all
'pages from doc1, doc2 and doc3
....