How to select a Relay that can handle a load of 42V, 3A DC?
The article you linked really neglects some stuff.
The contact voltage rating of a relay is effectively how far apart are the contacts. This limit keeps the relay from spontaneously conducting because the voltage can arc from one contact to the other. Do not exceed the contact voltage rating. The coil has its own separate rating.
The current rating of a relay typically has 3 ratings, sometimes 4. The AC current, the DC continious current, and the DC breaking current. Sometimes the AC breaking current is added. With AC the current constantly switches back and forth. This guarantees that the current will pass zero. When this happes, even if the contact are activly arcing, the current will stop. Typically the AC current rating is the same for continuous current and breaking current. DC is a whole other story. Relay tend to have high DC continuous current but low breaking current. Because DC flows constantly, if an arc forms in the contacts as they open, it may not extinguish. The lower breaking current is to make sure that the arc extinguishes.