Yes, it can be done.
First, check that there a common ground connection for all 3 thermistors. If there is, you can use 1 sensor board to measure 3 voltages with respect to that ground, all with the range 0 to +5v. You can make that sensor board using an ADUM6401 digital isolator http://www.analog.com/static/imported-files/data_sheets/ADuM6400_6401_6402_6403_6404.pdf and a 4-channel SPI ADC chip such as http://ww1.microchip.com/downloads/en/DeviceDoc/21298e.pdf. Conveniently, you can buy an evaluation board for the ADUM6401, which saves you making a PCB yourself. You can wire the ADC on the prototyping area of the board.
Not only does the ADUM provide isolation between the ADC and the Arduino, it also provides a 3.3V or 5V isolated power supply to the ADC. Note that there is a minimum load that you must put on the isolated power supply. I normally provide this load using an LED and series resistor, so that I can see that there is power on the isolated side.
A lower cost alternative would be to use opto isolators to isolate the ADC from the Arduino, but then you would still have to provide power to the ADC chip somehow.