Hello Susan, and welcome to the forums - glad you could join us!
All current (and recent) DSLRs are capable of very good low light results, considerably beyond the capabilities of your very good but ageing K100D. As you may already be aware, to get the best results you really need to shoot in RAW and do some post-processing in software such as Lightroom for colour and luminance noise reduction.
The K-3 (and K-3II) will produce excellent results up to ISO 6400. Depending on your subject, the level of luminance noise you can tolerate, and the amount of detail you wish to retain, you can get useable results as high as ISO 25600. The highest setting of ISO 51200 is really only of any value for very small images, and in my experience is best used for monochrome, if at all.
The new K-70 anecdotally performs even better than the K-3/K-3II at higher ISO settings, although the example photos I've seen thus far don't seem to show quite the level of improvement I'd expected. It's worth checking out though, for sure.
Regarding Canon, historically they tend not to perform quite so well as their competitors in terms of dynamic range (this will be most noticeable when trying to recover detail from shadows). From what I've read, very recent models such as the EOS 80D show a marked improvement in this respect, but it's something you'll want to investigate when looking at Canon's offerings.
Hope this helps
EDIT: One last point... I don't know what lenses you typically use, but - at the risk of stating the obvious - low light shooting benefits hugely from fast lenses. The wider the aperture you shoot at, the lower you can keep the ISO setting.
Last edited by BigMackCam; 09-24-2016 at 02:34 AM.