This is meant to attach to a spray wand that has a female quick connector on the end- so it won't work with the Karcher K3. The filter end goes into a chemical container, and the flow of water picks up the chemical, mixes with it and sprays the mix out through the tip.
Karcher is one of the few brands that will pull from a still water source, Kranzle is another if you're looking for a high-quality unit. This Yard Force unit has it's own water bucket and is handy off-road. You will see them described as "self priming".
This is the Karcher Suction Hose. The video is very informative.
The K3 Follow Me is a good unit for around the house or boat- washing a car or two; cleaning a small to medium size patio, washing off lawn furniture. It's not the best choice for big jobs like cleaning the siding on a large house.
Please remember that the self-priming action depends on a siphon- so you either need gravity from the bottom of a tank (as shown in the video), or if you're trying to pull from a lake you may need to start the siphon by sucking on the hose to get the water started, then adjusting the level of the pressure washer to keep the water flowing.
Neither the Karcher nor the Kranzle unit will "pull" water very far- the closer you can get to the water source the better it will work. If you lose the siphon action, running dry will ruin the Karcher pump, but the Kranzle pump will be fine.
I'm sorry, but I'm not familiar with Reliance pressure washers; we don't sell them.
I hope I've been able to help!