When a monster wields multiple weapons, and can attack multiple times with each, how do I alter (or should I leave) the given attack bonuses on each type of attack on the enemy's stat block?

Take, for example, monster Asura, Upasunda:

Melee: Mwk Longsword +20/+15/+10 (1d8+6/19-20), Mwk Spear +19 (1d8+6/x3), Mwk Kukri +19 (1d4+6/18-20) OR 6 slams +18 (1d4+6)

I'm pretty sure the OR means on a full round action I can choose the slams or all the others, but say I choose the weapon attacks to make a full round attack, after the first 3 attacks with +20/+15/+10 does the spear then get the +19 and ignore the usual attack bonus negative progression because its a different hand/weapon? Or what happens?