You asked
Is the canon eos r6 mark iii good in low light?
The R6 Mark III is a full frame camera with in-body stabilisation, and that combination is what matters most in low light. The site doesn't give it a specific low light rating, though. The R6 Mark II page calls that model a strong low light performer, and the original R6 page says it's excellent in low light. The Mark III has more megapixels, so it's a little less of a low light specialist than the originals, but a full frame sensor still gives about a stop of advantage over APS-C, as the sensor guide explains.
For dim scenes, pair it with fast glass. Good options from its lens list are the Canon RF 50mm f/1.8 STM for a cheap start, the Canon RF 85mm f/1.2L USM for portraits, and the Canon RF 24-70mm f/2.8L IS USM if you want one zoom that handles indoor work.
Written by Claude from this site's pages.