Depends on whether you're wanting to use MM1/2 monsters or make new ones.
If you're looking to use MM1 & MM2 monsters with little or no work converting them, then you want to divide their HP and multiply their damage (all of it, not just rolled) by 1.5 or 2. My experience has been that that MM3 is actually around a 1.67 to 1.75, but that's a lot harder to calculate in a hurry. A 1.5 is obviously going to make things feel closer to the old style and be less of a shock to your players, while a 2 is going to make combat very fast and deadly, and might not be a good idea if your players aren't running relatively optimized builds (my DM specifically allowed people to rebuild their characters a fair amount to account for the change after a session or 2 of trying it out).
If you're making new monsters, you can use the errata-ed monster creation tables found here (warning: that goes directly to a PDF); that should bring the DMG monster creation in line with the MM3 monsters.
Alternately, you can use the tables from that PDF and the damage expressions here which have been specifically maintained to keep monster damage high through level 30 (normally monster damage as a % of player HP drops as the players gain levels, with the most powerful damage expression, limited, going from about 55% of player HP at level 1 to about 33% of player HP at level 30).
Above all, remember that MM3 increased monster damage and reduced monster health. If you use the updated MM3 damage expressions but the old MM1/2 health values, you're making the monsters harder than they're intended to be.