This may be a wacky idea, but what about taking out a loan for a used Model S that you would sell when you get your Model 3? Would the S hold it's value enough that you'd lose less money than buying an selling a used Leaf? Would getting a CPO Model S move you up in line for getting your Model 3 earlier. Just a thought.
GAH! Don't tempt me!! The cheapest Model S is I could find now seems about equal (2013 for $46k). Assuming depreciation for 2-ish years, we'd be down to what $30k in 2018? It could make for a nice M3 trade-in....it's just a question of paying for that much car right now.