You can't, plain and simple. You need a total return of 150% over three years, which is about 45% per year plus compounding. Even the most riskiest of investments will not return 20% and you also run the risk that you will see a loss of capital (hence the high risk). At moderate risk, you are looking at a mix of stocks and bonds (most likely A or B+ bonds) to get you to 6 - 7% annually - assuming you get the 7%, you are looking at end of year 3 balance of about $250,000
You could do this with your own real estate business or any business where the product has a large price tag with a large profit margin. If you buy a fixer-upper for $100,000, spend $20,000 fixing it up and then sell it for $150,000 in a 3 month time period, then you've made $30,000 in three months. For a total profit of $300,000 you would need to do that 10 times or more since all deals won't be that good. But this is a manageable goal to accomplish within 3 years.
This is not what you want to hear. But I predict you are going to go bankrupt quickly if you continue you're current mindset.
Perhaps you do have $200,000. And I don't know how you'e managed to make it that far. With the way you speak, I don't think you've made that money yourself.
Anyway, why are you even on Yahoo Answers? I suggest you consult an expert.
I don't see any possible way to guarantee $500,000 in 3 years. If that was the case, I wouldn't even start a business, I'd just continue to invest it. Steve D is absolutely right, his answer basically says it all!
Hoping to grow $200,000 into $500,000 in three years STRONGLY indicates you lack any kind of business sense (or Common sense!) whatsoever, so you should probably "get a regular job", my innumerate friend...the expectation of a 75% annual return on a cash investment is insane...