I've done some basic conversions of monsters from from 3X/PF to 5th, but only the attack bonuses and saving throws and skills, when necessary (leaving almost everything else as is).
Going from 5th to 3rd, would be harder. Creatures with natural armor would need a lot more added to compensate for the PF/3X BAB's. Proficiency bonuses from saves would likely needed to be converted to PF/3X levels (IIRC, subtract two, multiply the result by one-half hit dice, then add two back for the saves the monster is proficient in). Attack bonuses would need to be converted. Ability scores would need increased, in some cases (especially when the score is 20 or more).
It would be some work to do an accurate conversion.