I've been working at company A for almost 3 years and they will be filing my 1st H1B extension in May under premium processing as my current H1 expires in August(am eligible for 3 years extension since I've only completed less than 3 years in H1B). However I found a job with company B a month ago which did a H1B transfer recently and is approved but I haven't joined them as my I-140 from A is still pending. In the mean time, I got an offer from Company C which I want to join in June/July.

Can I continue my employment with A without any issues/new transfers as I havent joined B ? Can I join company C after I get H1B extension from A and apply for H1B transfer from A to C? Does C need to aware of my existing transfer for B ? any potential complications that I need to be aware of ?