Agree with GD unless there is some restriction you didn't mention.
Mithral Gloves, Mithril Sword, and Dragonscale Shield are unique slots. So that's one Dragonscale and two Mithril without any haggling.
This leaves the Dragonscale Armor, Dragonscale Helm, Mithral Armor, and Mithral Helm.
Pick either the helm or armor in dragonscale and the remaining Miithral item for the last slot and you have your 3+2.
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