I could have had more than three references if I needed it- I didn't even use my dissertation supervisor (who is the HoD) as I felt there were people who knew me and my work better- all three were Profs, one is Vice-Dean of faculty and one is former HoD. In three or four years at university, you'll have to come into contact- through tutorials and labs/seminars primarily- a decent number of people. If you've only managed to impress two of them (I reckon Ive been taught in tutorials by about 12) sufficiently to write a short reference in your support, yet still think your application is strong enough, then there's something not right. At the very least the course convenor should have access to all of your work, and should be able to put something together even if they've never met you based on the quality of what you've done- but I'd consider that the last, rather than first, resort.