Serial port communication with multiple slaves in VB 6.0 via COM ports

Pretty much, you can't - not with standard COM ports.
COM ports connect via RS232, which is a hardware specification and it defined to be point-to-point where what you need is multidrop. PtP connections are only designed to support one device at each end of the cable, and while sometimes you can "get away" with putting three or even four devices on it, you have to have good control of your hardware from your software (which you don't have in VB) and a lot of luck with the hardware suppliers of your physical COM ports. Some will work, some won't. And you won;t know until you try them - and worse, some will work today, but not tomorrow ... because the weather is different, and that affects electronics once you go outside specification. Because the PSU is busy and the 5V rail is a little low. Because ... loads of things, all outside your control.

You need to look at a multidrop hardware environment - I'd suggest LAN or WLAN since they already exist in most environments, or go directly to via the internet / cloud.

But using standard COM ports? Doomed from the start unless you have one COM port on the server for each separate slave device, and that's a PITA to physically organise.