For a chemistry project; 9mg brexpiprazole + 700mg sertraline + 175mg quetiapine, what would that do to the heart of a previously healthy 20 year old man?

it depends not only on the dosage, but in the length of the treatment, it is not the same taking a monodosis than multiple dosis in a month. But for only one time it may cause taquicardia and also those drugs and enhance the probability of an arrithmya

When discussing effects of chemicals on living individuals that is much more than a chemistry project. Doses mentioned are higher than doses commonly used in psychiatry, too many unknowns for a straight answer otherwise.