Pacis Puella,
I think the reason you are getting so few responses is that it is a surprisingly hard question to answer. My first thought was that you could not say “may you have/be at peace from [your enemies],” the more I think about it, however, the more confused I am about it. It may be a case where we native speakers are going to give you the worst advice. As one person has indicated, to create a felicitious English construction you would ideally translate it without actually using the word “peace,” but since it is a gloss on the passage “pax tibi” you really have to.
In the context of 3 John, Diophenes was someone who was spreading bad blood about John in the church. If it is a gloss of John’s farewell where he is promising to come and talk about things face to face, is it possible he is simply saying “be at peace about Diophenes and all the troublemakers” until I arrive? I think “ab” could carry this sense in medieval Latin (defined as “in regards to.”) Diophenes wasn’t brandishing spears, just a wicked tongue. Then again, it could also mean “I hope they leave you alone.” “May Diophenes and all the wicked leave you in peace,” would be nice, but is perhaps stretching the Latin too much unless you have context to back it up. If you say “may you have peace from Diophenes and all the wicked” I do think the meaning will be clear since the reader will automatically presume that peace isn’t coming from the wicked people. It just sounds awkward, and possibly technically incorrect (while at the same time sounding like something someone might actually say . . . and possibly correct, heck I don’t know). I know someone who might know (for my own peace of mind now), and if I do, I’ll get back to you.
It sounds like it must be an interlinear gloss (since it is talking about Diophenes in the present tense). Are you translating both the interlinear and the Glossa Ordinaria? I’d be curious to know what books you are working on, which manuscript you are using (Rusch?), and if you are working towards publication. I’ve just been getting more familiar with the Glossa myself.
Sorry this isn't much help. I mostly wanted to give a possible explanation of why all of us "experts" in our mother tongue are not giving you much help.
Good Luck!