Synchronise in Premier Pro
Use In/Out points.
Put an "In" point on the audio clip, and one on each of the already-synced video clips. Those "In" points should be put on the same sync points (that is, moments that are clearly visible on all clips, say, the clap-board clapping and the sound of the clap!). Whatever sync-point you choose, make sure it's also visible on your non-synced clip. 
Put an "In" point on your non-synced clip, at the sync-point that you've already marked on the synced-clips. 
Choose to sync your sequence by "In" point.
The program will align the "In" points and you should have a synced sequence.