For a boost converter, (power out) = 0.8*(power in), with a margin for error. Some are a bit more efficient.
That means that a boost converter supplying 6V at 1 A, will draw about 2.5A from a 3V source. That input current can overload some converters so you have to consider both input and output current rating.
Pololu gives both, for their high quality regulators.