Here a method of making a voltage proportional to the current through the heater(s). I scaled the gain of the opamp to get 5V/5A, and put a trimpot in gain setting path, because the gain-setting resistors are likely to be no better than 1% tolerance. You will have to calibrate the gain.
Note that in order to use just a single supply 10V to 24V for the opamp, it must be the type whose input common-mode allows differencing of signals at ground, or even a few mV less than ground. An LM358 and similar ones allow that. If you were to use "normal" opamps, you would need to add a split +15V -15V dual supplies to run the opamp. Note the grounding method.
I was assuming that you needed three separate current to voltage converter channels, connected to three different A/D channels. If so, use a quad opamp, and build three of these circuits.