I've got an Android, the freebie I used is called gStrings, but there's quite a few free chromatic tuners out there for both Android and IOs. You'll want some earplugs, but you can then use the app to sound each horn individually, and it'll give you what the tone is in hZ to get it set back to after you pull it apart. Once they're close, you can do the last bit somewhat by ear, as they make a chord when fired together. It's been a while, so I don't know what each was tuned to anymore...the F# was just me humming for example.
You could also just record a sound clip of each one individually, too...then use that as a reference to tune it back in as well. Likely doesn't hurt to do both...belt and suspenders.
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