the box is needed to decode the cable signal from comcast. the only way to do it without is to split the cable coming out of the 1 box and then run a cable to each of the other tv's. this means you will need to set the channel in the one room and leave it alone once you're in another room. to change channels would mean getting up and doing to the main room with the box.
not an ideal situation but the only way. the encrypt the signal this way so you're forced to use their expensive box.
you can go away from comcast and use a streaming cable service like directvnow or playstation network or sling tv or youtube cable (whatever they call it). lots of options these days. saves a ton of cash since there will be no equipment fees. but you...