Since you have purchased this flat in August 2015, so you have held it for more than 2 years therefore it's a long term capital asset and you need to provide the value of purchase and sale so that I can calculate capital gain tax and other things.
However assuming you will have capital gain you will have two option to get exemption from such gain either by investing in some residential house u/s 54 or investing in various bonds listed in section 54EC of the act. However since you are saying you are going to invest in property I don't think it will save your capital gain tax and you will have to pay capital gain tax.
Regarding putting the money in capital gain account I would like to say that one need to put the money in capital gain account if they are unable to use the money of capital gain before the filing of return of income of the financial year in which they earned it and they say that they would use it to calim exemption and they don't want to pay capital gain tax. In such case one need to keep amount in capital gain account.
However if you are clear that you will pay capital gain tax and you will invest such amount in the property then you can keep the amount easily in your saving account.
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