The turbo 3-cylinder is one of the engines that comes in the clio and the micra, so it is possible that we get that engine, but other than it being a possible choice, Mitsubishi has not made any mention about it actually happening, currently it's all just rumors and speculation. The crossover thing came about pretty much entirely because the mirage is supposed to be on the juke's platform after it gets redesigned, but, so is the clio and the micra, so again, just speculation.
Long story short is pretty much all we know right now is that the mirage is still supposed to be a car, and it will likely be based on the cmf-b platform.