Hello and welcome to the RTA forums !

I believe that the average high in Vegas for March is in the high 60's so you could see 70 plus later in the Month. However, with 1 to 3 weeks and being prepared to fly across country gives you many, many options. The whole country and too many to mention in fact, so I would suggest you do a little research with a good map and a search of the forums to find the things that appeal to you and your interests. If the weather/temps are an issue you can normally find results by typing in place/month/weather to the search engine.

When you have decided what to do and for how long I am sure we can be of help and "fine tune" your plans.