Your equation would only be true if you invested 250 and never added to it.
But you're adding money monthly so that at the end of 3 months, you'll have 750.
Furthermore, is the account paying on the basis of the balance at the end of the 3 months? In that case, it's in your best interest to wait until the very end and deposit all 750. To avoid this, most investments that pay do so on the average amount in the account for the period in question.
You need to check the terms and conditions to verify exactly how they determine the base on which to pay the 40%.
Personally, I can't think of anything legitimate that could afford to pay 40% in 3 months. What could they be doing with your money that would generate a return high enough for them to pay 40% in 3 months? In this case, I'd be more worried about the return OF my capital vs the return ON my capital.