The tankless may demand more gas than both of those furnaces combined! You need to add up the BTU's required for all of the devices, and then you can go to a gas pipe sizing chart to determine what size the supply requires. But, as a guess, if they include a 3/4" inlet, my guess is that they expect that size pipe. The length of the pipe also plays into the required size. The instructions should define what is required for inlet supply line...do not shortchange it, or the performance will suffer badly when all are calling for heat at the same time.