Here is an explanation of how the two friends plan to kill the third friend.
One of the two tells the other to "get up in fun / To have a wrestle" with the third when he returns from the store. He is asking the second friend to engage the third in a friendly wrestling match. The first friend also says while they are wrestling that he will "up and put my dagger through his back." He then tells the second, "While you and he are struggling, as in game; / Then draw your dagger too and do the same." Therefore, the first friend plans to stab the third friend while he is wrestling with the second friend. The second friend will also stab the third friend afterward.
Basically, they will trick the third friend into participating in a wrestling match, and then both of the other two friends will stab him with their daggers. This reinforces the "with friends like these, who needs enemies?" moral of the story, especially since the third friend is off buying poison to kill the other two. The tale also emphasizes the affects of greed through the death of all three of the friends over the money that they found.
The quotes are from the Nevill Coghill translation.