Overtraining can happen but is not usually a factor with the less is more, less frequent training frequencies that are in vogue now, including yours of only twice weekly.
You're only training twice a week, so my first reaction is that there is plenty of time in between to add another session. Also, the plyometrics will take something out of you but are only to be done once weekly, which will help avoid overtraining.
Instead of asking and getting different opinions, all of which will vary depending on experience, age, individual differences, etc. you should try the 3-day thing for a while, a few weeks minimum and see how your energy and workouts go. Use that as the gauge; if after a few weeks or a month you're not too exhausted after doing this and feel you're getting good results, continue. Stop once in a while to assess, continue if the signs are good.
In general i'd say thrice weekly shouldn't be a problem based on your routine, which is not too lengthy, and the fact that plyometrics are only done once a week.