This itinerary is built up of 3x day trips. This means you always return to Reykjavik in the evening and head out on a new adventure the following day.
It doesn't matter which order you do the tours in. You can swap around as you like. I would recommend to check the weather at the start of your trip. It can be different conditions in the different areas so you may want to schedule the tours depending on that.
Rental Car
You need your own car to do this tour. I recommend Blue Car Rental, they have great cars at good prices. Click this link for 5% discount applied at checkout: https://www.bluecarrental.is/?affiliate=asasteinars