Although we've always supported the DIY ethos, you shouldn't try to make a triple wall chimney pipe by nesting pipes inside each other.
I'm using a slow-burn furnace/add-on woodstove, and I was wondering whether it would be possible to substitute ordinary galvanized heating duct — of 6", 7", and 8" ... or 6", 8", and 10" in diameter — for triple-wall pipe. I thought this might be possible, as to my knowledge all triple-wall pipe consists of three different diameters of pipe with air spaces between them. However, I'm not sure that galvanized metal could handle the high temperature, and I'm concerned that the coating on the inside of the pipe would be a problem. I also imagine that you'd have to shim the pipes evenly apart to avoid getting hot or cold spots. Is my idea feasible?
The homemade triple-wall chimney pipe you are considering could work for a while, in principle, but I strongly advise against your trying it. Your worries about the durability of galvanized steel pipe as a flue liner are justified. Although the melting point of the steel is adequately high, the galvanic coating could be melted and/or vaporized. The steel would then be susceptible to gradual high-temperature oxidation or burnout, and to acid corrosion from the presence of wet creosote.
In addition, you wouldn't know what clearances, from the outside of the chimney to nearby combustibles, would be safe. Listed prefabricated chimneys are all tested to determine safe clearances (two inches is the typical figure). And subtleties such as the amount of air leakage and flow at the joints, the thermal conductance of spacers or shims (which are necessary to avoid hot spots, as you guessed), and infrared emissivity of the pipe surfaces can all affect the outside temperature of the chimney. You'd also have to estimate and allow for the substantial thermal expansion (as much as 1/4" to 1/2" in a three-foot length of chimney) of the inner liner.
Therefore, I recommend using a prefabricated or masonry chimney rather than a do-it-yourself arrangement.