Some of the prairie seed companies have instructions in their catalogs and/or web sites. If memory serves, the basic options are: (1) herbicide; (2) strip the sod with a sod cutting machine; (3) smother (but three acres would be tough); (4) repeated tillage; and (5) sometimes a well-timed burn (depending on many factors and legality).

There is more detail required for all of these methods.
More experienced folks can give you better advice.

A nice controlled burn.

Check for permits and call the fire department. Offer to let them burn it for you and practice fighting grass fires at the same time. They often will do it!

Herbicide would be my 2nd choice.

I bought my mix from Michigan Wildflower Farm and their instructions were to kill each season's weeds. So that means hitting everthing that comes up with Round-up (or less priced equilavent at TSC) for a year. It's a huge pain and I hate using herbicides but it's so hard for the natives to compete with all our non-native weeds. There are tons of weed seeds waiting to spring forth once they're exposed to light.You gotta hit 'em hard.Once your natives have gotten some size they can hold their own. Then mow the crap out of your new planting starting early 'cause non-native grass gets growing way earlier than your natives and you want to keep your native seeds exposed to light. It's hard work but worth it.Start with small areas,otherwise it can be overwhelming. Good luck and don't give up.

You might consider goat rental, at least for the first pass to get rid of the tall stuff, unless you want to keep the biomass.

Use a grass-specific herbicide. The active ingredient is sethoxydim. It's sold under the name Poast or Arrest.
Just make sure some of the "good" plants aren't in the grass family.

Round-Up is great for the initial treatment, but you will have a continuous cycle of grasses as the seeds laying on the ground sprout and grow.