Question about Dell UltraSharp U2415

It is hard to imagine how the three units would have been connected at all with the supplied cables ;-)

At least as long as you believe the product descriptions. According to the product description, your PC has two graphics outputs:

1x HDMI
1x VGA

The monitors, on the other hand, have the following inputs:

2x HDMI
2x DisplayPort
1x MiniDP

Whereby (according to the description) only one DisplayPort cable is included. This would mean that nothing can be connected. But let's assume that an HDMI cable was also included, then you have at least already connected one monitor, which is obviously the case. That leaves the second monitor. This would have to be connected via VGA, because there is only one free VGA port on the PC. Since your monitors do not have a VGA input, a VGA to HDMI (or DisplayPort) converter would be needed first. But such converters are rather exotic products, so make sure you find one that can do more than FullHD. Because the monitor here is 1980x1200. This eliminates a good part of the converters. But item no. 11615177 could fit, for example.
Alternative: Get the graphics signal out via USB. There would probably be one or the other product.

So much "speculation"... if you want an exact answer, you would first have to write what you actually did ;-)

Well, 'David-cslu' has already written almost everything there is to write about it.

Another and probably the best solution:
Plug a graphics card with 2 Displayport outputs into the PC. This way you can connect the 2 monitors without having to use adapter cables.