At 3 Friends, "sustainability" means a

We are in the business of helping you see the world more clearly, in every way possible. For us, that goes beyond simply cleaning the dirt off of windows, but to every part of the equation that makes for a good view. We strive to be as environmentally friendly as possible, because that's what those windows are made for looking out to. 
We use 100% biodegradable, plant-derived cleaners, non-toxic to plants and animals, but strong enough to cut through the toughest dirt. 
Our towels of choice are recycled medical huck towels. 
Razors are recycled. 
Most homes can be cleaned using a mere 3-6 gallons of water. 

The primary vehicle is Jesse's VW Jetta TDI, which is run on biodiesel whenever possible, and averages close to 40 miles per gallon. 
If ladders aren't needed, it's not uncommon for us to show up by bike! 

Beyond environmental sustainability, we also are sustainability of our partners and .. 
The "friends" are a kind of partnership, and income is shared fairly, assuring a living wage for all.