If they are out of sync to start, you will need to do a migration job to update each of them.  Three separate migration jobs is probably quicker and easier than trying to do it all in one.

If trying to do initial setup and want them to be synchronized, there is a tick box on the Description tab of each part that indicates that the Master Part Description should be used on all display views.

It is also a good idea then to remove the permissions from the secondary part views to prevent most users from being to update the descriptions at all.  

The descriptions are out of sync, Can i use an expression to pick Master Part description in the migration job to update the value.

Yes, you should be able to do that, though it might be quicker to build a stacked migration job where the first one pulls only the part number and description to a temporary table and then use that temporary table as the source to update the descriptions everywhere.  This would of course only be viable if you are sure that you want the master to overwrite all other versions of the description, meaning that somehow along the way the inventory part or sales part description hasn’t become the better one because someone was updating them in the wrong place. 

That to me is the tricky bit in all of this is determining whether there is any good info in the other parts that needs moved up to the Master Part first before then bringing the entire lot back down across all of the other parts.