Purslane! I've always had it growing in the cracks of cement and was surprised to find it was good to eat!I used to call Sorrel "shamrock" because that's what it looks like: the leaves are parted into 3 leaflets, each one sort of heart-shaped. There's a low-growing, creeping plant that grows commonly in my area in addition to sorrel:
Purslane. Is this it?