your strategy is only possible because of zero electricity, otherwise you would earn much less per month, because the consumption for all those old amd are very high and kill half if not more, of the profit
currently you can make $2k per month(this require accurate selection of the coin you mine, private trading, some times holding etc...) with 2 rig of 1070, so i have your same ROI but i also pay electricity....
your constant shilling of 1070s is making me want to buy some of that LOL.. with the release of 1080 ti, enthusiast gamers are selling for an upgrade and an enthusiast miner (me) is enticed by the price from these second hand gear.
OP's zero electricity is his leverage to jump start his mining venture...getting the highest possible hashrate for the cheapest hardware he can get, after ROI and beyond the free 3kw, he can start with the efficiency strategy..imagine him buying 1070s one at a time from profit, and long term he might sell those old cards and upgrade to much newer and efficient gear.