I would have called the vet to see if they could stitch it, but that doesn't mean that it won't heal with what you have going now. I don't think the vet can stitch it at this point given it's been 3 days. Given that, it will scar, not look pretty, and take a lot longer to heal. Depending on your knowledge and experience you could close it up on your own without a vet. I estimate a month to 6 weeks of diligent cleaning and bandaging.
I would look at a few different options as far as treating it given the horses personality and tolerance, and what supplies and medicines you have on hand.
First thing is you should hose it off or flush it off with some water daily for the next few days to remove any debris. Flush it for a good 10 minutes.
Secondly I would dab it gently with dilute betadine to keep it clean. Don't scrub, just dab.
Third I would make a "bandaid" out of gauze/tefla pad and elasticon, put some antibiotic ointment on it and put that on to keep it covered and clean. Don't put it on tight, just loosely and covered/sealed completely. I would do this for 1-2 weeks depending on how it looks, then reevaluate. I don't mean wrap elasticon around her whole face, just make a square bandage from it. Rub the elasticon inot the hair to get it to stick on.
If that didn't stick then I would try either a salve(like sylvadine) over it to help keep it covered apply 1-2x a day, or I would use wonder dust as a last resort.
Lastly I would keep a fly mask over her face, but buy one of those long ones that won't rub on it, maybe one of those with a wire in it, and keep it on until it's closed.
I would do that to start, then check on the flap of skin, and if it begins interfering with the healing process have it removed so it can continue healing. I have personally done this with a scalpel, but only if your horse is going to let you.
Once it starts closing up and gets to the size of a dime or smaller then I would stop using the bandage and switch over to just wonder dust, keep the fly mask on until it's completely scabbed over. Getting to this could take 2-4 weeks, but after the wound starts draining less in the first week or so you can leave the bandage on for 2 days, then 3, but I wouldn't leave it on longer than 3 days.
Hopefully you will not have a fight with proud flesh here.
Good luck, I've seen much worse! You can do it!
PS. You can also super glue wounds like that in the event you can't stitch or staple it. No personal experience on that one but I've got a friend who's used it to close and cover cuts.