SAL is the easiest to transit and has a lounge. Of the three airports, it is the one that regularly sees connecting passengers.
MGA and TGU are nothing to write home about. Both are quite small, even relative to SAL, though both are more than adequate for a short layover.
As for TGU, it's more common to fly a straight-in approach to RWY 20 than to do the "YouTube landing." So if you choose TGU strictly for the approach, you could be disappointed. It is also the most prone to weather cancellations.