A 1S battery is recommended to minimize the voltage drop across the T3's LDO linear regulators. A 2S battery will work, however, the voltage drop increases across the LDO when used, manifesting in the form of heat (so they will run hot).
This is especially so for the T3 Transmitter, as it's much more current intensive than the receiver.
Receivers require much smaller operating current and will run cooler.