While you work on this with her, you might want to keep the toys put away and occasionally block your dog in a bedroom/outside so that the other dogs can have playtime without her. (And of course, she could have her toys too while separated.) That way they don't miss out but you don't have to worry about her behaviors getting reinforced or a fight possibly breaking out if another dog reacts badly to her guarding.
When I adopted my previous dog, she had some intense food guarding issues. The vet told me to start walking by with her favorite treats as she was eating and toss one in her bowl each time. The idea was that she'd start to associate me approaching her food with getting even better food, rather than seeing it as a threat. It worked really well for her and she stopped lashing out at me during meal time. The vet mentioned that sometimes people suggest the opposite approach of "showing the dog whose boss" and that can just make it worse; if you're nervous about someone taking your food and they keep reaching over and grabbing stuff from your plate, they're kind of proving that you had something to worry about. It's about fear/insecurity for the dog - they're scared that something they place a high value on will be taken away. After teaching her that I didn't mean any harm by getting near her food, my dog was able to relax.
The vet offered a similar suggestion for toy guarding, though thankfully my dog never got as extreme about that. Offer to "trade" her a treat or a better toy for the toy she's guarding so she associates you taking it with something positive. And if she gets weird about her toy while playing, turn your back and walk away so she knows that getting defensive ruins the fun. I would imagine you could alter those tactics a bit to apply to guarding from other dogs as well, but I just had the one dog and just had to worry about her guarding things from me. I don't want to try to suggest anything outside of what the vet specifically recommended to me since I'm certainly not an expert.
I'd recommend asking your vet for advice about it since they've no doubt seen this sort of thing before, and don't hesitate to get a professional involved if you feel like your dog may really hurt someone. Since the habit seems new, hopefully you can stop it early. Move as slowly as you need to and put your safety first. Good luck!
I would go the safe route and not have toys out while your dog is with the others. You just don't want the worst case scenario to happen, especially when you're already aware of it. It can be a bummer, but maybe you can separate the dogs so they can play with their toys without you needing to worry.
I would agree with Cassandra with keeping toys put away or separating toy play time! I do this and it works effectively.
I had an aggressive dog stay over Labor Day weekend. She would growl show her teeth when a certain family member would come into the room where the dog was I would put her back in crate until she calmed down then I felt bad.she was a lot of work for the money I should have been getting more. For wha