only have 3 days to travel and will be driving a small ford focus
You can take a look at the route that Trippy generates for Vegas to Chicago as a starting point. It spreads out the trip over 4 days, but you can always skip a stop or drive faster (especially on the third day) to do it in 3 days.
Click on the link that says "VIEW DETAILED MAP" to see a bigger map that you can browse. Basically, the route takes I-15 to I-70, then I-76 to I-80 and that pretty much gets you the rest of the way.
The last stretch of Nebraska and Iowa might be less interesting, so that's where you could drive longer stretches to cover more distance when you're getting tired on the third day and you just want to get to Chicago.
Good luck with your trip planning!