First a few notes on this.
* First of all setting the heapsize to 19GB means java will reserve 19GB of memory for each instance you run, i.e. in this case 19GB x 3 which might no be what you want.
* Secondly using that big heap size is likely to affect the performance of the test since java will take time to build and manage the heap and should be done with caution.
With those two things said, How does your ram utilization look in task manager while having the problem?