I would say it would be ok assuming the tank filter is cycled and it circulates water well, but if the platys breed you could be in trouble
However, i would personally pick either lemon or rummynose tetras and get 8 of them. Schooling fish like the company of their own kind. Corys will be ok in a trio but i would ditch the platys and get 3 more corys as once again, they are a schooling fish.
I would also recommend a sand substrate, not gravel, especially not sharp large gravel as the corys can damage themselves when digging for food.