How many ESP32 WiFi devices can transfer data to an android phone same time?
... that's only limited by the number of devices that can be in the same IPv4 network that you put these devices in, and the number of WLAN devices your access point can deal with. So, something between 2⁷-2 to 2²⁴-2 devices.
I need to transfer 3Mbps of data from each ESP32
Aahh, different question!
So, the fastest standard supported by the ESP32 is IEEE802.11n, which, under ideal circumstances (point-to-point, 2 devices only) can do 150 Mbps raw data.
Including headers, non-ideal geometry, and the fact that multiple devices trying to access concurrently will greatly reduce that potential number of average transfer rate, you'd be happy if you still got 50 Mbps across, on average.
That'd be 18 devices streaming 3 Mbps. It's a bit optimistic, and assumes that you can always stream large chunks of data, so make sure there's enough buffering on each ESP32 device to allow for that!
Again, "being connected to the server" and "actively streaming 3Mbps, simultaneously" are two very different things.