It does seem to me that there are many Pagans who would reject the concept of three-fold return based upon thinking it is a form of reward and punishment. My understanding of this concept varies from that, however. First of all, it is not necessarily three-fold. That is simply the number applied to the concept. In reality there is no way of quantifying that, and it can likely vary from one situation to another. My understanding is along the lines of throwing a stone into a body of water and having the ripples extend outward to touch (and affect) the rest of the world. Once one understands the interconnectedness of us all, this is not a difficult concept to understand.
This principle also acts in mundane terms and can be explained through the following example. We all have choices to make in life, and even the smallest ones can have an impact. When we walk down the street and come in contact with one of our neighbors we have a choice to make regarding how to interact with them. (Even choosing to ignore them is such a choice.) In this example I will limit the choices to two. We can either smile and cheerfully greet the neighbor or we can frown, stick our nose in the air and walk by without acknowledging they exist.
These may be relatively inconsequential actions, but they hold potential for great impact. If we respond in a cheerful manner we might brighten an otherwise difficult day. If they come in contact with two other people and are more cheerful toward each of them as a result, the effect upon each of them might be similar to your effect upon your neighbor. The converse would be true had you chosen to snub your neighbor and unpleasantness would be getting spread instead. If we take this one step further, we can imagine each of the two people upon whom your neighbor had an impact having such an effect upon two more people and on and on. Eventually, the pleasantness or unpleasantness would come back to you.
Some of you might be saying to yourselves at this point, "I thought that was karma? You know...what goes around comes around?" I believe that is not how karma works. My understanding of karma is based upon the concept that we are here, experiencing an earthly incarnation, for the purpose of learning. My understanding of karma is that the universe presents us with opportunities to learn these lessons. Usually the universe will give us a situation in which we can learn the lesson the "easier, softer way." If, however, we are the type to "need a house to fall on us," it will. Hence, the term "bad karma." It's my understanding that by not learning the lesson at the first opportunity, we will be placed into progressively more difficult situations by the universe so that we must take that lesson. This is why we have a saying about strength growing from adversity. Of course, "good karma" would be a situation where we have already learned the lesson and are able to easily deal with a particular situation.