You can usually parallel the outputs of DC supplies but might have issues with different types. Getting the current shared equally is one. A bummer with high current supplies like you mentioned, is that it takes a ridiculous amount of capacitance to filter the output. This is especially true of a linear supply with 120Hz ripple. The 3 supplies would do well for you if they were fed from three phases and each had full wave outputs. The ripple frequency would increase but the ripple amplitude would be greatly reduced.
Joe
