A good match, perhaps from the Zürich area...

[Koen G]

Could be percussionem, I guess.

That's also my best guess.

I scanned a few dozen more pages but haven't found any "c-3" yet. With the evidence we have though, I think Marco's suggestion of "lucent" is a reasonable one. Cappelli doesn't have "c-3" but does have two examples of "consonant-3", which are transcribed as -ent, in contrast to -et.

   

The "apparent" is a broader abbreviation, but the principle appears to be the same. The first example "debent" is a good parallel of how we'd get "lucent".