Tarahen ,
At a former employer, our Onboarding process was very involved, lengthy, and complex. We actually surveyed employees for all three of those timeframes. We gave them a heads up that they would be receiving one survey a month for their first 90 days. Each survey was different and asked them different questions based on that phase of their onboarding process. There was a long learning curve and so this approach was useful.
If your company doesn't have as complex of onboarding, perhaps aiming to have the survey sent out 1 week after the onboarding process is completed would be ideal. That way it is all still fresh in their head and can give the most applicable feedback.
As a new employee I like receiving surveys from my employer. It helps me know that they are listening to me and care about my experience. So if you have a short onboarding process and send one after 30 days. I do not see anything wrong with an additional "check in" survey 3-6 months in as well. But only if your team has the bandwidth to do something in response to the feedback given.
Hope this helps.

For a client I am working with we opted for quite early in the process at 14 days after hire.  This captures the fresh new employee sentiment and sets the stage that the company wants to hear from its employees.

Hope this helps!

As others have mentioned, it will likely depend on your onboarding process.  You’ll want to send when the onboarding is still fresh in the employee’s head.  Typically, I recommend 30 days if that makes sense for you, then again at the 6 month mark to make sure things are still going well.