To find the ratio of almonds to chocolate chips, we divide the number of cups of almonds by the number of cups of chocolate chips: 18/34 = <<18/34=0.5294>>0.5294
Then we divide the number of cups of chocolate chips Luis uses by the ratio to find the number of cups of almonds he needs: 3 cups / 0.5294 = <<3/0.5294=5.6701>>5.6701
Rounded to the nearest tenth, Luis needs 5.7 cups of almonds. Answer: \boxed{5.7}.