Multiple Shipments from one order

Hello,

We have a sales order that needs to be split into 3 shipments.  They are all going on the same day.  We start with a single shipment number and use P4915 to remove lines from the shipment.  That puts them all on a second shipment.  How do we get some of those on a third?  If we try to remove they end up being added back to the first.

We've tried to create a new shipment number manually but can't seem to get it to find any orders to select.

We are tools / apps 9.1

Any help would be appreciated as this is something we will need to do frequently.