Lucky you- it should be in the 70's in March.
Have a look at the Weather Underground site, you can find out the weather history for your dates

I went in March a couple of years back. Weather was pleasant without being hot. We took a light jacket to wear during the day walking about. We went to the Grand Canyon and there was a few inches of snow there so do take some warm clothing if you're going out there.

Cheers,

TC

Thanks for the replies