As a general advice, when a building is in fire, people inside should go outside, and stay at an open space/ground/field.
Which word should I use?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
All three seem a little odd. "Open space" is probably the least odd. It means "land that isn't used intensively" and would include parks, fields, woods, and undeveloped land in a city. But do you really mean that the people should go to a park?
"Open ground" means land that is flat and free of buildings completely, right outside the city. And open field would mean a flat agricultural terrain.
You don't actually need people to stay at an open space. You want
"Go outside and stay outside".
(It is okay to repeat "outside" for emphasis). If you need to say that they can't stand close to the build that is on fire then
"Go outside and stay away from the building".
If you need them to avoid all buildings (as might be advice after an earthquake.
"Go outside and stay away from buildings".
If you have a specific place to go to, such as a park called "Kings Park":
"Go outside and muster in Kings Park".