@NoGi
What is your strategy? If you want to check if a topic has not had any posts after 3 hours and then take action, you will need to schedule that process to happen for each topic after it is created.
First, create a custom action in the rules ACP and add an \IPS\forums\Topic as an action argument.
Then create a rule that is triggered by topic creation. Make sure to add a condition which checks that the boolean value "new" from the event is TRUE (since the same event is also triggered anytime a topic is updated). In this rule, add the action to schedule your "custom action" for 3 hours in the future. Use the created topic as the argument to your custom action.
Finally, create another rule which uses your "custom action" as the event. Since your custom sction will be triggered for a topic 3 hours after it was created (because of that first rule), you add your conditions to check the number of replies on the topic and take appropriate actions here.