I would be exhausted by 3 museums in one day and also suffering from sensory overload. Both the Shedd and the Field Museum are huge; the Shedd also has shows of dolphins and whales at certain times that you might want to catch. How would 6 Y.O handle all that in one day without a meltdown? Only you can judge.
I would just buy tickets online from the museum websites.
To get there you could walk over to Michigan Ave and take a 146 bus southbound. Take one that says "Museum Campus" on the front. Or taxi or uber if you are in a hurry to get there.
Yes, they are next to each other in the Museum Campus south of Grant Park. You can see them all in one day if you don't want to see much of anything at any of them. Choose 2 and relax. If you are buying a city pass just to use everything on the pass (with a 6 year old along). Don't let a citypass force you to see what you don't want to see. Do the math.
You can take the #146 bus from Michigan Ave to the Museum Campus.
You can certainly walk through them all in 1 day. You will miss a lot but if you are checking a box have at it. I love the Shedd and FM they take a 1/2 day min IMO. For a first time visitor... even longer
I love museums but seeing them all in one day would be hellish, especially for a six-year-old.
They’re also all expensive. It makes much more sense to pick one and spend the day there rather than rush through, The planetarium is small so if your 6 year old can last all day you could do that plus the aquarium. But all three is just rushing to say you did it.
I'd do the Field and call it a day. If the 6 has energy after the Field then you could go on line and buy a Shedd ticket. Note that not all 146 buses go to the museum campus -- it will say Museum Campus on the front if it does and it is a great way to get to the museums. I once made the mistake of getting on the wrong 146 and it was a longish walk from the last stop over to the museum. The Museum Campus bus will drop you right at the museum. The route ends at the Adler -- but I would never try to do all 3 in a day and especially with a child.
There is pretty good food service at the museums.
Fully agree with all the above responses (sure you could do all in 1 day, barely scratching the surface of the Field and Shedd; but more recommended to spend more time in Field or Shedd).
How long is your visit and what else do you have planned? The CityPass is valid for 9 days so, unless the rest of your trip is packed to the gills, or you only have a couple days, you have plenty of time to make a couple trips to the Museum Campus.
I don’t know if you belong to a museum at home, but our season passes allow us in free to Adler and Field. If your kids are like mine, racing through the museum, I’m sure you could do three in one day if you wanted.
That’s a lot in one day. Perhaps 2. Try to enjoy the experience. We always take the #146 bus to the museum campus.
I did the Shedd and Field in one day with a visitor who was determined to squeeze every dollar out of one of the city passes he bought, never ever again. I’d prioritise the Field, Adler and Shedd are fine but they’re not particularly unique, especially Adler. I always think of Adler as school field trip when your science class is studying astronomy.