You asked
Is the canon eos r3 good in low light?
The site doesn't give the EOS R3 a low-light rating, but the specs point the right way. It has a 24.1-megapixel full frame sensor and in-body stabilisation. On the sensor guide, a full frame sensor has roughly one stop of low-light advantage over APS-C, which is why full frame is the usual choice for dim events.
For dark scenes, pair it with a fast lens. The Canon RF 50mm f/1.2L USM and Canon RF 85mm f/1.2L USM gather a lot of light. If you want one zoom that covers more ground, the Canon RF 24-70mm f/2.8L IS USM is a solid choice. If you're comparing bodies, the Canon EOS R6 Mark II is described on its page as a strong all-rounder for events and low light.
Written by Claude from this site's pages.