You asked
Is the fujifilm x a3 good in low light?
The site doesn't give a low-light rating for the X-A3 itself, so I can't tell you how its high-ISO files compare with other cameras. What I can tell you is that the body has no stabilisation, so for slow shutter speeds in dim rooms you'll want a stabilised lens, such as the Fujifilm XF 18-55mm f/2.8-4 R LM OIS.
Lens speed matters more than the body here. These fast primes on the X-A3 gather much more light:
For why a fast prime helps, see Primes or zooms. For whether full frame would be worth the extra size in low light, see Is full frame worth it?
Written by Claude from this site's pages.