The Precision 3660 should support up to three 3.5" hard drives and the max capacity is the current capacity available.  It's easy to add three 16TB to your system.  Two internal bays and one at the front flex bay. 
I am looking at a Precision 3660 Tower to use as a small home server.  I want to add three SATA drives in addition to the single SDD.  However, looking at the specs am I correct that there are just two bays for SATA drives and the max capacity of the SATA drives supported by the 3660 is 4TB?  I was looking to add 16TB drives.  How easy/difficult is it for me to add three 16TB SATA drives to an "out of the box" 3660 Tower?  Thanks!
Precision 3660 Tower Hard Drive specs