okay, I'm going to try to illustrate why this is a bad idea.
First off, if you get the lump sum all at once, you don't get the benefits of dollar cost averaging. This basically means that if you happen to invest it at a time right before a big downswing, you can end up getting in big trouble. Dollar cost averaging works in your favor.
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Second off, your reason for being able to beat the market is really really naive. So I'm going to go ahead and assume at best you will match the market. Historical returns show it's about 8-10%, right? Then when you factor in taxes, you lose another 2-2.5% (assuming 25% tax rate) so it's like 6-7.5% yield (assuming historical returns). You are BORROWING at 12% to earn money at a rate of 6-7.5%. That means you are netting a -4.5 to -6.5% effective yield by using this strategy. That's assuming all goes well and you are matching historical returns, which not that many investors actually get.
Sure, you have a possibility to come out ahead, say if you outperform historical market returns by a decent amount. But how much do you have to outperform the markets by to make this a +EV decision? You need to BEAT 12% in
after-tax dollar returns on the stock-market over a 3 year timespan. Just how hard will this be? Well you need to get returns of 16% on the stock market (assuming 25% tax rate) over the 3 years TO BREAK EVEN. If you are confident you can get 16% on the stock market, then you should go down to wall street and get a job in some sort of i-banking or hedge fund type job. You could be making millions of dollars if you knew how to pick stocks that get 16% over the long haul. There's a reason that job pays so much, because almost NOBODY can do it.