just know the following moves.
1. use an item safely. in MH3, if you use an item you most certainly will be targetted.
2. from a sheath position to a weapon charge stance (hold L+X). essential for great sword and hammer users.
3. dodge roll. left, right, forward and backward roll especially in a unsheathed stance.
4. from an unsheathed stance, know when you can safely sheath your weapon while NEAR the monster and RUN! useful for dodging large area attacks.
5. the superman dive! while running, do a dodge. it makes you invisible for a few frames. funny to see other player doing this online.
6. don't spam attacks. it is ineffective and will get you killed. each weapon class has a certain uses. Great swords are charged to level 3 before attacking, it is slow but gives the greatest damage. Hammers, charge-pound on the head. Lances, well poke-poke-poke-side step... poke-poke-poke-side step... with the occasional poke-poke-poke-counter. Charging with lances is fun with lances, but do it online and everyone will hate you because its disruptive.
7. learn to focus on the monster using the right-thumb stick as they tend to go to your blind spot quite often.