I’ve used sets of 100 in the past (see my HSS-100 article). They can help with recovery especially if you are using a proper peri-workout protocol pre and during your training: for ultra high reps to work you need to have nutrients already in the blood since the ultra high reps work mostly by increasing blood flow to the muscles and if the blood is loaded with nutrients you have a much greater effect.
You only need one set, 3 is too much: you will just burn more glycogen without much increase in nutrients uptake by the muscles.
Soreness is not a good indication of your recovery status nor is it a good indication that you had a good (or bad) workout. However inflammation soreness could be a sign that it’s best to way before training muscle again.
Not likely… 100’s have little impact on the nervous system, which is the no.1 reason for overtraining from lifting. You can have overtraining via too much volume but this happens mostly from endurance training… and in bodybuilding/lifting high volume has to be combined with neurally intense work or work that leads to a chronic inflammation to possibly cause overtraining, and 100’s aren’t in that category.
The only way to « cause overtraining » by adding 100’s would be to be on a very very low carbs/calories diet…. but then again it isn’t really the 100’s that would be the main cause.