This policy always troubled me for a couple of reasons:
1.-A lease by its very nature, involves only one recipient. Imagine if you had leased a Lexus for your spouse, and one evening some guy showed up with a set of keys, attempting to take the car. His response is, "I have a lease for this car as well. Right now I am in need of it." I'm sure that wouldn't fly too well with you.
2.-These smaller carriers which permit multiple "leases" are generally operating on the overflow from other carriers. These companies are generally looking at the same loads. How does it benefit you to have three carriers bidding against one another to get you the same load? All it means is that the carrier who is awarded the load will make a bare minimum to secure it for you, insuring that your fuel surcharge will not increase anytime soon, no matter what the fuel prices do. If they can't make money on the load, neither will the truck!
3.- Lastly, I've met a few van driver who subscribe to this philosophy. I cannot say that any one of them are very successful with this paradigm. The fear is that they will sit, and that three carriers means three times the opportunities. More likely, it will mean that you are never able to build enough of a rapport with any carrier to cause them to favor you.
In my opinion for what it's worth, you would do better to find a carrier that you trust (which takes time), and that will work as hard to load your truck as you do for them. I'm amazed that there are some companies who watch for loads, while others will SEARCH for loads.