I-140 and H1B extension

1.If one changes employers before approved I-140 is revoked and has applied for 3 years of extension based on I 140 with previous employer, then he is fine, right?

2.I-140 can(/may) be revoked after he has transferred h-1/changed employement.

is he allowed to get more extensions after that or he needs to again file new labor and I 140 with new employer to get three more years extension (based on revoked I-140)
(once he already shifted to new employer based on three years ext on previous 140)?

3. How could check if the I-140 is revoked or not personally?

thank you