We know that we need to bake 6 pound for 138 minutes, then we have the proportion:
Let x be the time we need to bake the 3 pound roast, in this case we have the proportion:
By equating both ratios we have the proportion:
Now, by simplifying the right side of the proportion we have that:
For this proportion to be equal we need that x=69, therefore we need to cook the 3 pound roast for 69 minutes.