The craziest thing is that that single external display can be up to 6k/60hz! Surely for those 21.2 million pixels you could run 2 4k displays at 8.2 million pixels each.
You get back ports, but those base model M3 MBPs only support a single external display. You get HDMI and 3 40gbps thunderbolt ports, but can only use one of them at a time for screens.
You need to bump up to the M3 pro models to get multiple external display support.