Why would my house be so humid even when the A/C is running?
A few theories as to what might be going on:


You have bad ductwork with few returns. If that's the case, you're just not moving enough air to dehumidify every room. 
Your AC unit is oversized. And oversized AC unit will cool the air very fast and then shut off. The problem with that is if the AC unit is only running for short periods, it's not given enough time to properly dehumidify the air. 
You have massive air leaks in the house. Maybe as simple as an open door or window. Maybe more complex like no insulation or air barriers in the walls. 
You have a massive water leak in the house. Leaking foundation/crawlspace, leaking pluming in the wall, roof leak, etc.


A 1700sf isn't a huge house, so I have a hunch a 3 ton unit is simply too big for your particular house. On average, do you know how often the AC kicks on and for how long? Do you know if they did proper calculations on your house before sizing the unit?