Your concept is an interesting.

Providing,
- I have 1 BTC.
- ETHBTC is 0.045
- I am buying (margin) 1 / 0.045 * 3 (leverage) = 66.6 ETH.

My questions are:

1) who will sell me 13.5 BTC?

2) my position is marginal. Let's assume ETHBTC became 0.050. My profit is 66.6 * (0.050 - 0.045) = 0.33 BTC.
Who pay me my profit (0.33 BTC)?

3) providing the seller is the one to pay my profit (or to profit on my loss). What if the seller has insufficient funds?