That is not completely true. It really depends on two things, what their definition of dual outlet means, and how tightly regulated the supplies are. Many high current supplies have multiple outlets but they are all connected to the same common point. They have the multiple outlets so that they can power more than one device without putting the full load on one set of wires and if they are connected to one point inside of the supply, there is no issue connecting them back together on the outside.
Dual rail and separate supplies are a bit different. Even in a dual rail supply, there is no guarantee that the voltage output on each rail is going to be exactly the same and it is even less likely with separate supplies. If the voltage difference falls within the regulation specs, then you can connect then together although they may run a bit warmer than normal. If the voltage difference is outside of the regulation spec, then they can begin to fight each other which will cause them to get warm or even hot .... up to the point of failure (little puffs of smoke). Since it is difficult to determine up front if these separate rails or supplies are within spec of each other, it is often recommended not to connect their outputs together.
Three outputs and zero resistance between the three positive wires leads me to believe that the three wires are all connected to a common point in the supply and if this is the case, then there is no issue with connecting the outputs together. Dual rail supplies have a very high resistance between the positive leads of separate rails since they are basically two separate regulator sections sharing a common high output source and connecting these outputs together could have unexpected and sometimes poor results.
