We are there the exact same dates! Partial day 15th, then full days 16th- 18th!
We are going to go into Disneyland on the 15th, with no rush, no plans, just to take in the fact we are really there. Then starting DL 16th, DCA 17th and DL 18th, we do have parkhoppers.
I don't know how it will be given it is holiday time, but from what I have seen from vloggers the weekdays have actually been just as busy, if not busier than the weekends, due to the combination of pricing differences (for single day tickets) and passholder blockouts. Trend for a while has been Sunday is quietest and Monday is busiest!
Obviously I have no idea if that will hold for the holiday season, and I have no personal experience of it, just what others have reported.
Hope you have an awesome time!