Hi Arms
How much of your equity you can use will depend on the loan to value ratio that the lender is prepared to loan you and the value of the properties that you now hold. eg
Say your three properties are worth 1,000,000 and your current loans are $700,000 (leaving you $300,000 equity) If the bank is willing to loan you 80% you would have an extra $100,000 ($1,000,000 x 80% = $800,000 less current loans $700,000 leaves $100,000) you could use for a deposit and costs on another house. Obviously if the bank or other lending institute wanted a different loan to value (LVR) ratio this would change the amount you have to invest.
I would recommend talking to a good mortgage broker who understands investment property to advise you on how to structure your current loans to be able to use the most equity.
Silas