I use a Hoya NDX400 (9 stop) filter on my Pentax 16-85 in Bulb mode, with a Vello Wired Remote. The remote allows me to use any amount of time I require to get a reasonable exposure. I set my ISO to 200, maybe an F-Stop of F13, and test with different intervals of time (seconds, minutes) to narrow down to where I get the exposure I want. Before I shoot, I press my LV (Live View) button and use in many instances Autofocus if I see in Live View that the results are satisfactory. If I notice that I do not like the AF result, then I can use Manual focus with AF turned off. Live View AF during the day has no problem on my K-5IIS using my ND filter. You may want to set your white balance to Daylight also. Make sure to cover your viewfinder with the viewfinder cover to avoid light, or shade it with your hand. If you need a wired remote switch, the following is the type I have, which requires no battery due to it deriving its power from the camera. It is listed as being compatible with the K-3 along with other Pentax models.
Vello RS-C1II Wired Remote Switch for Select Cameras RS-C1II B&H