I believe Mothballs.. or episonsalt's. Try your local Home Depot or handy store.. I think theres something called snake-b-gone ' i think'
However its only last for a while. My mother's very scared of snakes in genral. I use mothballs around her garden and haven't see a lick of grass snakes or black snakes.
At my home (lake front) I have to resort to a shovel due to the fact that I live in the city limits...
I get those water moc's.. these fuggers will chase you lol!! It's been a rare seasion I haven't seen but one snake.. I wonder If I have killed htem all or the mothballs are working here too
Least you can shoot the sum'iches

On a bad note: "You" need to look your place over well. 3 in one night you have have a nest or something that is attracting them.