"What would happen if I used the Vcc 3 volts and the data lins Rx/Tx are putting out +5?"
Tx won't put out 5V, it would only put 3V.
If the other end drove Rx to 5V, then the Rx pin would get damaged - the uC only tolerates Vcc + 0.5V. 5V is well above 3.5V.