I use RPi 3B because of lower power consumption than 3B+, 4B... It is around 300mA. I connect power cable to USB from router that has 500mA output. It works great.
RPi 3B+ has sometimes problem with stability because of higher power consumption.
Official power supply of RPi is 2,6A if I remember correctly, so 3,5 is not even possible?
Btw, many routers support sleep mode. It is not necessary to run router/connected RPi 24/7.