I have my I-140 approved with H1B validity until 2019 May. If i change my employer now,

1) Do my new employer immediately need to file Labor or can I get my extension done with my approved I-140
2) If i can extend my H1b approval with already approved 140, how many years will i new H1b extension be, 3 years ?
3) What happens if i get my H1B extension gets denied under the new employer peition? What are my options ?