A.B.
I am a Medical Transcriptionist, but I can give you some pointers on billing/coding. The most lucrative of the 3 is coding, followed by transcription and then billing. You do need a certificate and there are many avenues to accomplish this: on-line schools, trade-style schools, community colleges. I received mine from a local trade-style school called Techskills and you can get any of the 3 certifications from them. Do some research and figure out what you want from the school. Some give you a specific amount of time to complete the program and if you do not, they start charging you more. Techskills has one cost and you can take all the time you need to complete it.
Transcription is the best choice, if you want options to work from home and flexibility. Billing and coding pretty much has to be done in an office environment. I am sure there are part-time jobs for billing & coding, I haven't looked into that too much.
I plan to go back and get my coding certificate in a year or two, so I can make a bit more money when my kids are all in school full-time.
Hope this info helps.
AB