The one that sounds better to me is 3, because 'last resort' usually takes an indefinite article. The only exception I can think of is the phrase 'lender of last resort' (and I've only heard that in to places - first, in an Economics lesson in the late '60s, and more recently during the Northern Rock crisis nearly a year ago - BBC NEWS | Business | Timeline: Northern Rock bank crisis - when much was made of The Bank of England's role as 'lender of last resort'.
But I wouldn't call 1 or 2 wrong. And this BNC search shows that 'lender' is by no means the only possible noun in this collocation - although it's by far the most common:
1 LENDER 29 404 7.18 9.20
2 BANK 5 17596 0.03 3.67
3 ROLE 5 17993 0.03 3.64
4 COURT 5 28470 0.02 3.18
- which shows that my personal preference isn't everyone's (which some of you will probably have guessed by now. )