We have a long commute to school and I am interested in moving beyond the standard Kiddie Rock or Kindie Rock fare for DD3. She likes music a lot and I want to expose her to a variety of good (but child appropriate) music. Any suggestions?

For the commutes, starting at age 2, we moved exclusively to our local listener supported classical radio station. They play all styles of classical music interspersed with many trivia and information on composers, conductors, instruments and the stories behind pieces, information on the cities and the times during which the pieces were composed etc thrown in.
The funny thing is that DS is very musical and takes several music lessons (2 different music instruments, theory, ensemble etc) - and whenever one of the teachers has tried to introduce a composer or a popular (or not so popular piece), DS chimes in with a lot of related information spontaneously and they are left amazed at how a young child knows so much about music history and related topics. He has developed his own taste for music through this. And it is like background music during the commute so that DS can talk to me when he needs to without the music being intrusive. So, it might be an option to look into.

Broadway musicals are a lot of fun, and you can sing along. You can get soundtracks or "kiddie chorus" versions. How about some Gilbert and Sullivan? Also consider folk music, and kiddie folk like Trout Fishing in America (they also record for adults). Wheels on a Big Rig and 6 were huge faves in our car (as well as "all I want is a proper cup of coffee made in a proper copper coffee pot")

I like ABBA for the kids, lots to sing along with and very complex musically. We tend to play a very wide variety though.
One thing we tried for a while was giving each week (day) a theme, so 50's rock, classical, folk etc. That was fun and easy with things like Spotify.

Now the kids make their own requests. Number 1 Abba, number 2 The Ramones. (I'm blaming their father for that)

we recently bought a cd including Tchaikovsky, The seasons. He has a song for every month of year. My kids like to listen and guess about which month it could be.

Just do what you said, expose her to all kinds of music. Be careful though, we had a period of being stuck with South American pan pipes as a result! Make up a playlist if you have the facility to play that from a phone or tablet. Rotate CDs or program in a variety of radio stations. Ask her to guess the genre, explain what makes something country, classical, jazz, rock or pop. Ask her opinions. There are albums of music for kids that aren't kiddie pop. I remember some with popular classics. Don't expect her to like anything straight away. Just like with adults, tastes change with exposure and over time. Mine loves jazz and some rap where I don't really but I love when she develops new and interesting tastes.