Hi @Tingw16594322: You can achieve this in multiple ways. I see you have a solution with you now. If you are looking for a solution to achieve this in a single step, it is not possible. Even with Activity/ Validate rule, you need to define the validation for all the 3 fields with the condition. So it is better to create a when rule for this condition.
Create a when rule and configure to required condition in the section. Also configure the same validation using validate rule of the flow action. One issue here is that the section configuration will just show the message as this field is required.
Create a when rule for the condition, Configure validate rule in the flow action for all the 3 properties
You can also do this in the post activity. But it is recommended to do using flow action validate rule