Not exactly sure what you’re trying to accomplish, but I assume you wish to continue to next API call based upon with one finishes. Inherently your microflow will execute in order, so if you have one single microflow that does 3 API calls, it will wait until the first one is complete before moving on..
Alternatively if your API calls are called asynchronously, look into using the TASK QUEUE to manage retries and string together calls by only calling the next API at the end of your task queue flow.
If you need to, you can use a scheduled event to check completion of an event as you suggested but you will need to store API call completion details somewhere in a entity for your scheduled event to look at which may not be ideal.
Would using a task queue be an option for you? https://docs.mendix.com/refguide/task-queue/
You can set threads, interval and number of retries.
Or are there more specific requirements i am not aware of?