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Not a transformer. Transformers only work with AC, not DC. You could use a boost converter (using an transformer instead of the regular inductor would make the voltage get stepped up even more). But 3 to 5V is such a small step-up that a regular boost converter works just fine....
BUt the simplest thing is to use a capacitor charge pump IC made to turn 3V into 5V if your current draw is low enough.
Oh...if you're using this current to power a motor you shouldn't regulate it at all. It's a waste (of parts ane energy) to regulate voltage for things that don't need it. A 3V motor is probably fine on 5V depending on how much you are loading it. Or just stick a power resistor in series with the motor to reduce the voltage a bit. The amount it's reduced by varies with current, but it's not critical for motors to run. The more the motor is loaded, the more current it draws, the more voltage the resistor drops, so the lower the voltage getting to the motor so it still balances out fairly well.
For those sizes there are very few differences in motors. The hardest part is coming up with a gear ratio to allow it to drive the blower fan for your skirt at JUST the right speed to optimize airflow vs current. It's not really an electronics question at all.
You guys are not listening. He said that the only thing that the 5V will power is the gyro.
Go to Linear.com. They make IC chips to create what is knows as a "boost switch-mode converter". In addition to the chip, it takes a few resistors, one inductor, a high-speed switching diode, a couple of capacitors. Typical efficiencies are about 90%, so if you need [email protected], you will be drawing about (5*0.2)/(3*0.9)=37mA off the 3V battery.