import win32com.client

ppt = win32com.client.Dispatch("PowerPoint.Application")
ppt.Visible = True
presentation = ppt.Presentations.Open(r"C:\Work\PPTCombined\first.pptx")

for f in [r"C:\Work\PPTCombined\second.pptx", r"C:\Work\PPTCombined\third.pptx"]:
    presentation.Slides.InsertFromFile(f, presentation.Slides.Count)

presentation.SaveAs(r"C:\Work\PPTCombined\merged.pptx")
ppt.Quit()