Mark,
WDW is huge w/ lots of walking which tires little ones out pretty fast. My 5 yr old never uses a stroller but loves to ride in one at WDW. I'd recommend you have a way for all 3 DDs to kick back & enjoy the ride. Single & double strollers are available for rent in the parks but can't leave the park so at the end of the day you could end up carrying any DDs not in strollers until you get back to your resort.
I prefer bringing my own stroller. The ones for rent in the parks are hard plastic, not very comfortable & not conducive for taking naps. Go to
www.allears.net & search for strollers to see a picture of them. I like that my personal stroller has more storage space than the rentals & I don't have to unload it to get back to my resort.
For 8 days, its cheaper to bring your own. Single & double strollers are available for rent ($15/day or $13/day for a Length of Stay Rental for a single & $31/day or $27/day for a Length of Stay Rental for a double - prices are subject to change w/o notice). Availability is a 1st come basis & can't be taken from the park in which they were rented. You can take your receipt to the next park on the same day & get another stroller (if available).
Here are a few stroller tips: Never leave any valuables in your stroller. There are designated parking areas for strollers at attractions & Cast Members rearrange strollers for max use of space. Tie a bright ribbon on your stroller to help spot it more quickly when you return. Put some form of ID on your stroller. Bring a stroller fan, extra batteries & a rain cover. A white towel over the canopy helps reflect the heat in the summer months. Don't leave open food in your stroller - squirrels & birds are very bold when searching for food. Wishing you & your family many magical memories :-)