FlexLogger can't currently run multiple instances on a single PC to run three different tests with one chassis each. Today, this would lead to resource conflicts between the separate instances of FlexLogger. (Note that a single FlexLogger project can run a test acquiring data from three different chassis at the same time.)
Today, your best options is to use three PCs to control the three tests. We are always glad to get feedback on important use cases. I don't have any news to share today about multi-head testing capabilities in FlexLogger, but we are regularly updating FlexLogger with new features that allow you to solve more problems more quickly and easily.