In the 3D Zeldas, is it faster to roll or to simply walk?

Yes it is always faster to roll. By how much depends on the game, so let's break it down a bit. All of this information was found on the ZeldaSpeedRuns website, which has in-depth mechanical information compiled by the community of Zelda speedrunners. All of the values provided are in units per frame.

Ocarina of Time

In Ocarina of Time, your standard movement speed while walking is 9 units as adult, and 8.25 units as child. Rolling and backwalking have a movement speed of 13.5 units as adult, and 12.375 units as child. As far as I can tell, these values are the same in Ocarina of Time 3D. Source

Majora's Mask

In Majora's Mask, walking as Link has a constant speed of 5.5 units. Rolling as Link has an average speed of 6.583 units. However, in that game, the fastest regular movement by far is spike rolling as Goron, which has a constant speed of 18 units. As far as I can tell, these values are the same in Majora's Mask 3D. Source

The Wind Waker

In The Wind Waker, Link walks at a speed of 17 units and rolls at a top speed of 26 units. As far as I can tell, these values are the same in The Wind Waker HD. Source

Twilight Princess

In Twilight Princess, Link walks at a speed of 23 units, and rolls at a top speed of 32.9 units. However, Wolf dashing is generally considered to be faster than rolling in most cases, which has a top speed of either 33 or 45 units, depending on the area. As far as I can tell, these values are the same in Twilight Princess HD. Source