As Dreams is a US based company how does it work if you want to stay on site for 3 weeks and have a two or three week ticket for the parks? I cannot see an option for this on the site.
I would email them - they are usually very quick at coming back to you I think they will work out a bespoke package for you. Usually they email you the price with a password so that you can go back to it when ever you like.
You are under no obligation to order your park tickets when you book your accommodation with Dreams, unless you book an inclusive package. You can book what ever tickets you want
If you plan to stay on site for 3 week, then I would look at an Annual Pass - you can then go for the AP room discounts and Disney Dining Experiance discount card (20% off meals).
This is what I've done with them - I've already got my tickets (days left over from previous trips), so I've asked DU to give me room only prices on our WDW hotels. They've certainly come up trumps so far on our 2007 cruise - we saved over 15% by booking through them!