```python
from pm4py.objects.process_tree.obj import ProcessTree, Operator

root = ProcessTree(operator=Operator.PARALLEL)
purchase = ProcessTree(label="Purchase", parent=root)
receive = ProcessTree(label="Receive", parent=root)
pay = ProcessTree(label="Pay", parent=root)

seq1 = ProcessTree(operator=Operator.SEQUENCE, parent=purchase)
seq2 = ProcessTree(operator=Operator.SEQUENCE, parent=receive)
seq3 = ProcessTree(operator=Operator.SEQUENCE, parent=pay)

purchase.children.append(seq1)
receive.children.append(seq2)
pay.children.append(seq3)
root.children.append(purchase)
root.children.append(receive)
root.children.append(pay)
```
This process tree represents a Purchase-to-Pay process with three activities: purchase, receive, and pay. The sequence of activities within each process tree is represented using the `SEQUENCE` operator.