The sand doesn't absorb or dilute the solution, so you would state it as 1mL per 1.5 cups of water (360 mL), which is 0.27% by volume. Although I'm not a biologist, my first concern is that this seems like a dreadfully high dose of Atrazine. I'm afraid you may kill the crabs immediately, which (apart from bad science) is bad stewardship of our resources on earth. You might want to pre-dilute the dose by measuring 1mL into a cup of water (0.42% of 240 mL), then measuring 1mL of that dilute solution into the crab tank. 0.42% of 0.27% is 12 parts per million (12 ppm).

Also, Atrazine has been reported to cause cancers, and it is absorbed through the skin. Only perform the experiment wearing rubber or vinyl chemical-resistant gloves, wear goggles and a lab coat (or an additional jacket which you will wash separately), and be extra careful that no droplets or spills may contaminate surfaces which you will touch or breathe later, and don't let any go down the drain. Be sure to get the Material Safety Data Sheet (MSDS) and read it carefully for handling.

I'm not sure what you're asking - you mean that there was a quantity of 1 ml of Atrazine for 3 cups water? I don't see what's wrong with just saying that with one exception: a ml is a metric unit and a cup is an English unit. I suggest you convert cups to metric. One liter = 1.08 quarts, and a quart means four cups, so that should be some fairly basic multiplication and division.

Contaminants in water (like the herbicide that you're adding) are most often presented as concentrations in water. In your case, you could say 1 ml Altrazine per 1.5 cups water. The volume of a cup is most often used in cooking (in this country, anyway), but scientists usually use volumes of liters (L) or milliliters (mL). I suggest you state your concentration in terms of mL Altrazine per L water. If you stick with 1.5 cups of water, that's about 0.355 L, giving you a concentration of 1 mL Altrazine per 0.355 L water, which is just a little bit more than 2.8 mL/L.