回复：回复：After filing transfer to B, he may start working for B. -

When you work for 1st employer and a transfer to 1nd employer is pending, you may file another transfer to 3rd employer. Once you have received receipt for 3rd one, you may starting working for the 3rd.

During transferring to 3rd, no such option to specify from 2nd or 1st on I-129.

If your H-1 for 1st is still valid, you don't need a bridge. So you may consider you are transferring directly from 1st to 3rd, with nothing to do with 2nd. Otherwise you need to maintain the bridge (make sure no problem with 2nd).