I could be totally underestimating the draw of a cars electric system or how much you drive, but a "few" deep cycles plus the starting battery seems excessive.
However much power you draw, its sure to be less than the max output of the alternator, which usually aren't more than 70 or 80 stock. Even a small group 24 deepcycle is 70-85amp hours, so absolute worse case scenario a single battery should get you an hour of driving with no alternator - thats assuming headlights, stereo, heater fan, everything on at once. If you replace all running lights with LEDs you remove 5-10 amps, and if your can has separate DLRs or can be retrofilled with them, LEDs will shave another 5-15 amps off of power needs.
On my system (and its a diesel, so no coil to power - but I do have a 200watt stereo, electric cooling fan and brake pump, and use the starter alot due to EOC in the city) the couple times I drove several hours at night, forgot to recharge at home and then drove again the next day the most I ever drained the batteries to was about 65% which is too low for a starting battery but a deep cycle can go as low as 20% w/o hurting it.
The only disadvantage to deepcycles is that they don't have as high of maximum amps (CCA) so might possibly not be strong enough to start the engine on a cold day - but if you had 2 in parallel the maximum CCA would be both added together.
In other words, if you have 2 of them, I can't imagine you needing the starting battery at all, never mind a 3rd deep battery.
If the alternator is switched and you monitor with a volt meter, as long as it has the CCA to start the car, I can't see why you would need more than one. Just keep it above 50% charge, (which you may never go below depending on the size of the battery and length of the drive), and top it off at home.
They are after all rather expensive and very heavy. All that extra weight will take back some of the mpg gains of the alternator delete, so you don't want to get anymore than you need.
If you do go all out though, you can get a switch to choose between 2 battery banks (bank 1, bank 2, both, or off) at any marine or RV store.
I found this site helpful when I was shopping for batteries just yesterday:
Deep Cycle Battery FAQ