You've filed a police case, so a compliant exchange should respond to freeze requests by legal entities (or so). Though, as I've never cared much about HitBTC, I don't know how and if they comply to such inquieries. I've fuzzy memories that HitBTC could be one of those less compliant exchanges, but my vage memory could be totally wrong here.
User accounts on exchanges usually have a receive address to send funds to your account. These are addresses of the exchange's hot wallet. Once funds have been received and confirmed, the exchange usually uses some internal database for bookkeeping.
Excessive funds on hot wallet are transfered to exchange's cold wallets for security reasons. In your example those three 20BTC chunks could've been UTXOs of the cold wallet. User's balances of funds in the exchange's internal database don't show up on the blockchain in a transparent way or at all. You only see in- and out-flow from the exchange on the blockchain at least.