3B has Wifi B/G/N, which means theoretically, a max of 54Mbit connectivity. If you are running Raspbian
Desktop instead of Raspbian Lite, you will not get full speed out of it. If you could ensure each camera connects to seperate APs, that would be of help.
The real problem is processing power and PI bandwidth.
My setup is Spectrum ISP to cable modem to Ubiquity EdgeMax Router.
I use a script to enable/disable the port forwarding on an as needed
basis.
I have several WiFi AC1750 & AC1900 routers set as Access Points.
I
have 3 D-Link 5020L WiFi/Ethernet cameras and 1 PiZero (running
motionEyeOS) CSI PiCam all the time, and 2 other setups (One
Pi3B+motionEye on Raspbian with attached USB Camera) as test beds.
I
have a virtual machine on a Virtualbox Server running Ubuntu 18.10 and
motionEye as my security camera hub. I have a NAS and the VBox Server
acting as storage and backup storage.
I have a Gigabit
Ethernet network internally (Fiber and Cat6 copper)(2x CISCO 2960X 48+2spf+POE). Multiple PCs,
ethernet and wireless BlueRay and Tivos, Smart TVs, Game Consoles, etc,
all playing nicely together.
I get an average of 10fps streaming per camera, and my D-Links are only 640x480.
The 3B+ gets about the same framerate via Wifi (AC) and ethernet. I believe it all comes down to it's only a PI. I can't recommend any of the competitors boards, they aren't as stable or have the support. I believe motionEye, as good as it is, is not really a competitor to any SecurityDVR, but is great for hobbyists and tinkerers.