SnS: Extreme mobility, good sharpness (If I'm not wrong, the sharpness is actually 1 level higher than what they show for SnS), able to evade most attacks, relatively fast, can both cause impact and slash, pretty good status effects, block if neccessary. Only weakness is low damage output, short range compared to the rest (though the lunge attack has good range) and don't expect the block to do miracles. Able to attack while hiding behind the shield (though you will drop your guard for a split second), and use items while guarding and selecting the item (Do not need to sheathe to use item).
GS: Highest raw damage in-game, okayish mobility, great at cutting tails, charge fer extreme damage and able to block as a last resort. Again, don't expect the block to do miracles, and you lose sharpness per block. Also unable to run, and takes time getting used to, due to slow attack speed. Great weapon to master, but be mindful in multi-play.
LS: High DPS, high damage and sharpness, able to run, good attack speed, spirit guage adds even more damage and sharpness. Mobility is great too. Only weakness is that it is a pure assault weapon, with only rolling for defence (no blocking). Good range too, beware of knocking down in multi-play.
Lance: Good reach, great damage, newly added charge attack and shield bash make it versatile, and the jousting attack (not sure whether it's still in MH3) is great for running slow, large monsters through, though I don't recommend spamming it. Not very good mobility, uses backhops/slides insted of rolling, shield is able to shrug off most attacks. If I'm not wrong, you're able to attack while hiding behind the shield too, like the SnS.
Hammer: Rivals the GS in terms of raw damage, has the potential for highest DPS weapon in-game, capable of causing KOs, and able to move while charging. A great weapon, if you can get pass its rather slow speed and short reach. Also a pure assault weapon with only rolling for defence. Note: The standard triple pound WILL put the hurt on anything you successfully connect the combo with.
This are all my thoughts on the melee weaps in MH3, can't say anything about the SA since I haven't used it yet. :V