I've looked at a lot of meshes that have been made at my work, and 3-sided tris aren't totally avoided, especially when you are modelling highly realistic stuff that needs bevelled edges and the likes. However, meshes that will be subdivided and characters are always kept to quads (as much as is possible), but it doesn't matter when your making a purely polygonal model. 5 or more sides should be avoided though. Sometimes its just not feasible to create a lot more geometry just to get rid of a few tris.