That regulator is appropriate for powering your Arduino UNO, but not appropriate for powering those servos. When you supply power from that regulator to VIN on the Arduino and supply power to your servos from the 5V pin on the Arduino, you are still using the regulator to supply power to your servos (it just gets regulated again from the VIN voltage to 5V by the Arduino's onboard regulator). Unfortunately, we do not have a step-up regulator that can handle much more current from voltages as low as two AAA batteries connected in series, especially if you boosting up to 9V.
Standard size servos like the one you are using can generally draw about 1A at stall, and it is good practice to pick an appropriate supply that can handle that in case the servo stalls. For a system like yours, I recommend using a 4.8V or 6.0V battery pack to supply power directly to your servos, and you can use the same #791 regulator to supply power to VIN on your Arduino.
By the way, powering your servos from the regulated 5V supply on your Arduino is not a good practice. While it has been working for you in your tests, if either of your servos draw more current (for example, if either of them is stalled), it could permanently damage your UNO.