Op amp to supply current to 3 thermistors and an ADC voltage reference input
Well, you have 800 ohm in series with each thermistor so, at the minimum thermistor resistance, the op-amp output sees 3 parallel resistors of 1300 ohms = 433.33 ohms and, if as you imply, the reference voltage is 2.5 volts then, the total current to be supplied by the op-amp output is 5.8 mA. I don't think this will be a problem to most op-amps providing that the 2.5 volts isn't close to their lowest output voltage when the negative supply is at GND.

The LT1499 can supply that.