2015 MBP (Thunderbolt 2) output to Thunderbolt 3 Dock
Get a proper Thunderbolt 2 dock.
The problem that you’re going to have is that Thunderbolt 2 does not provide all of the bandwidth and capabilities necessary to support what’s on the Thunderbolt 3 dock.
Using a USB-C dock with older MacBook Pros
Those Thunderbolt 2 to thunderbolt 3 adapters are great for when you’re using a single use items like an Ethernet port or a Firewire adapter. Thunderbolt 3 has double the bandwidth of Thunderbolt 2 -  40Gb/s versus 20Gb/s. You simply will not be able to support everything that the newer dock supports.
What’s also important to keep in mind is that the Thunderbolt 2 uses an active cable where as Thunderbolt 3 does not. So right off the bat trying to connect this dock to your older MacBook Pro will likely not work.