Welcome to Pentax Forums momof4
Just doing a google search it looks like you'll need a 58mm filter for the Sigma 70-300mm F4-5.6 APO.
Although I might be wrong.
The 18-55mm lens requires a 52mm filter, like Peter said.
When you have a filter on at the 18mm side you'll notice that the corners are dark
This is normal for the lens. It's called Vignetting.
Sorry I can't help you with the Pentax 500mm f/1.4 lens.
I think that you may have added a digit. I'm pretty sure that it should be
Pentax 50mm f/1.4 lens
If you bring the lenses into a local store they'll try on the filters and make sure that you get the right ones.
If you plan on buying I guess that this won't work.