Could I use a heatsink to interconnect transistors mounting base?
Yes, you can use the heatsink as a common drain for all transistors if your heatsink is not covered with paint but:
- It is mandatory to use a screws with washer in order to make a good contact.
If you use the circuit outside or in a humid environment probably after few months will appear some oxid on your heatsink and the contact resistance will be bigger (can rise until your aplication will stop working).

Another disadvantage of using the heat plate as only way of conductive part is that your MOS will have a greater response delay. If in datasheet it is written 5ns-tr, you will have more than 10ns because the layout is designed for a fast response when voltage is applied to the pin and the drain is connected to pin not to heatplate.