Bourbon Naples in 1815

I've been struggling with the Neapolitan military post June 1815 (I think that's when the Sicilian and Neapolitan armies were combined). Specifically, were there more than three foreign regiments, or just three battalions in one foreign regiment?

I'm trying to see if there was actually something called the 3e [Terza] Reggimento Estero or whether the eyewitness actually means 3rd battalion ...

I ask because there was a garrison of 50 of these soldiers at nearby Monteleone when Murat landed at Pizzo in October 1815. Some of the officers from this detachment were involved in his trial.

All this is building to finding out which unit it was that composed the firing squad ....

Any help appreciated.