H1-B Visa Extension with Approved I-140

I have an approved I-140, approved in 2012 from employer A (worked there from 2011-2018). I changed my job to employer B in 2018. I got a 3rd extension until 2021 with employer B when I changed my job in 2018. If I change to employer C, will it be 3 more years of H1B based on previously approved I-140? Employer B has not filed for my green card yet and I am getting nervous. My 1st H1B started in 2011. Can i change my jobs with previously approved I-140 with extensions for 3 years each time?

Also have your Employer B (or any other employer that you move to later) file AC21 for your i140.