One can almost say that there is no such thing as "Thunderbolt 3 over USB-C", because Thunderbolt simply changed the physical connector it uses from mini DisplayPort to USB-C since version 3.
So the question is rather, whether a USB-C port is a Thunderbolt 3 port, or whether a mini DP is a Thunderbolt 1 / 2 port, and that pretty much depends on whether a Thunderbolt controller exists on a motherboard and whether a port is routed to it.
The spec of T305CA listed a "USB TYPE C" port, while that of T303UA listed a "Thunderbolt" port (USB-C "physically", which makes it a TB3). That actually looks to me that T305CA does NOT have a TB3 controller. But if it does, then I see no reason that the USB-C/TB3 port on it cannot provide DP 1.2 connection.
Thunderbolt 3 (and 2) supports DisplayPort 1.2 connection, which supports 4K@60Hz. However, this is merely referring to the capability of the Thunderbolt controller. The actual display resolution that it can provide ultimately depends on the graphics unit that the Thunderbolt controller is routed to, which is probably always the one in the CPU.
For the T303UA, all the CPUs it may use support 4K@60Hz (in fact the actual maximum is slightly higher, which is 4096x2304@60Hz, while "4K" usually refers 3840x2160 these days):
http://ark.intel.com/products/88193/Intel-Core-i5-6200U-Processor-3M-Cache-up-to-2_80-GHz
http://ark.intel.com/products/88194/Intel-Core-i7-6500U-Processor-4M-Cache-up-to-3_10-GHz
P.S. Apparently there is "pure" DP over USB-C solution that does not involve a Thunderbolt controller. I am not sure how common it is on PC though. Maybe for routing a port directly to a dedicated GPU that is not involved hybrid graphics (which is itself a rare case these days)? Since AFAIK Thunderbolt controller cannot get access to a dedicated GPU directly.
