How to process multiple transactions of a user at the same time

A user is doing multiple transaction considering opening balance and closing balance of wallet. Using API he is doing more than thousands of transaction in a minute. The problem arises is 3 transaction starts at same time considering millisecond (ms) also and ends at the same. So each transaction has considered same opening balance which result errors in calculation. How can i handle this situation? I hope you can understand the situation. If you have any solution please share.