@Cuong Nguyen , I believe what @Mark Gallagher is referring to is using leverage (meaning, getting a loan) to cover the rest of the purchase price.. simple, yet not.
So, for example --> Property cost $100k. You put down $10k (of your $30k), and finance the remaining $90k. You do this three times and you've got 3 properties..
The reality of this is that unless you put down enough, the property won't cash-flow for you. (do the math) Plus, unless you're a seasoned RE investor, banks (most, not all) will want 20/25% down, unless you're living in it. So, the simple idea of using $30k to buy 3 houses might work for some, but not all.. and probably not a noob.
But, to answer, or at least give a response to @Ryne V..
With $30k today, I would use a portion to buy a 'live-in-flip', and hold the rest as reserves. Once the live-in-flip was completed, I'd refinance the house (hopefully getting a good appraisal-> CF & PMI and all of that) that would allow a decent amount of cashflow. Then take the remainder of the reserves and go do it again. I'd aim to buy a house every 8-10 months.