Add day3 solution.

day3.cr

@@ -0,0 +1,29 @@
+INPUT = File.read("day3.txt").lines.map(&.chomp) 
+
+def run(input, slopes)
+  prod = 1_i64
+  slopes.each do |slope|
+    trees = 0
+    pos = 0
+    line = 0
+    right, down = slope
+    while line < input.size
+      trees += 1 if input[line][pos] == '#'
+      pos = (pos + right) % input[line].size
+      line += down
+    end
+    prod *= trees
+  end
+  prod
+end
+
+def part1
+  puts run(INPUT, [{3,1}])
+end
+
+def part2
+  puts run(INPUT, [{1,1}, {3,1}, {5,1}, {7,1}, {1,2}])
+end
+
+part1
+part2