This trio of warriors is a completely optional fight, so don't worry about taking them on until you're ready. They wait for you at the end of the Cave of the Dead.
Obviously with three characters, you're going to want to even the playing field. Use the summon signs at the start of the zone, then make your way to the boss room. Once here with your own team of three, you can have a good, clean fight.
By "good, clean fight," I mean a 3-on-2 battle. See, Cerah the Old Explorer will hang in the background, firing his dragonbow. Lead the other two opponents out of this line of sight, then use your one-man advantage to pound them into submission--backstabs are your best friend.
With the soldier and graverobber down, make a final charge toward the explorer. Even if your summons are defeated by this point, the explorer is easy to defeat on your own. Just fight him as you would any phantom, and the battle won't take long.
For more help be sure to check out our complete Dark Souls 2 Boss guide.