I thought toads needed some more moisture around. I didn't know they woud burrow in the sand.

Toads, or at least the ones here, don't live in water. They will go into the water but prefer land. They don't hop but will crawl.
I have often found them make a burrow under a rock or slab where they stay until it gets cooler at night. Also they don't 'croak' like frogs but make a kind of 'squeek'. This burrow is about 18" from the fountain with the mint I have a photo of here.

Be happy they don't croak. A friend of mine's neighbors used to have a large frog-infested pond. When those frogs start (late in the evening, all night if they feel like it), it has nothing to do with the nice music-clip of 'we all stand together'. You need earplugs then.
I hope that squeek you mention is not too loud (three of them should be OK though).