Where I am you would be expected to make sure fencing was in order (repair/replace damaged sections and loose fence posts), that water was available (fresh clean water from a stream or from a tap - the horsey people are responsible for filling water buckets for the field and stables themselves, unless you have self filling troughs attached to the mains which you will maintain).
If they are using your stables that they were in good repair (wind, watertight, locks work), that if electricity is supplied (it doesn’t have to be in cheaper diy facilities, reflected in livery charges) that broken switches won’t cause shocks etc.
Practically, also you have to consider space for their cars to park, they may ask to park a trailer. There is also hay, hard feed (grains, comes in 20kg bags like dog food) and bedding (straw or wood shavings) which needs to be stored. A horse can eat a bale of hay in 24hrs. Depending on horse size/local availability of fodder/what storage they have at home they may be happy to buy a few bales a week and bring them up in a car. Or they may ask to store a tonne at a time to save money/hassle, if you have a barn. Or a spare stable.
If you have a tack room (room for storing saddles etc) it should be secure so their insurance is valid in the event of theft of their saddles. Otherwise they’ll keep those at home.
3 acres isn’t that much for three big horses. Three ponies would be ok. But the field will get poached (dug up by hooves walking/running on it in the wet) and this can turn it into a sea of mud. Some yards therefore put down a hard surface like stones or tarmac at the gates as these are the most travelled over areas. Some make the horses remain stabled over the winter or use another more crappy field for daily turn out.
The horsey people are not usually held responsible for anything they damage. So stable doors being booted in, fields a quagmire, doors and fence posts chewed to death, rylock fencing cut to get a stuck hoof freed, etc will end up your problem.
Lots to think about! If it was me I’d try and get one family with a couple of ponies and let them use this spare stable for their feed and bedding. Saves on fighting between liveries (not just the horses) and loads of cars parked all over the shop!
You won’t be expected to do anything else. Unless you choose to offer services for a price. Good luck.