Of course it depends upon what condition and the rarity of each card involved.
But, assuming all cards are in the same condition and all are of equal rarity, I feel that 3:1 is a fair exchange because of the difficulty in obtaining the blank cards.

Be advised that imprinted cards CAN be erased using a solvent like GoofOff. All of the screen printing is removed, but a slight impression is left in the plastic, which is visible by looking at reflected light at an angle. SO BE CAREFUL WHEN TRADING.