Will I be able to get a three-year extension with employer B if I switch because I have 140 approved from employer A?
Yes, as long as your employer is able to show that they have specialty occupation work available for 3 Years
I am getting my six years completed around same time next year so my worry is if I don't get extension based on approved 140 from previous employer A, I may have to go back
Any Employer can file an H1B Extension using an I-140 from another Employer
Also, my wife, H4 EAD is in process from last two months with employer A which has H1B validity 3 year extension but if I switch now to employer B with whom I have got only one year extension with transfer, not sure if She will get one-year or three-year EAD and if She get one more year what will happen to EAD extension ?
She will follow your H1B Approval duration since H4 EAD will go with H4 duration.