Let's be clear about this.
It is the company's b******t 3 years before command policy that is causing the problem. If you had a simple seniority based system, the fleet management could go down the list offering 310 commands. Eventually there will be enough guys who see an advantage to a quick command who will take the offer.
What we have here is a bunch of F/O's who are forced to wait a minimum of 3 years. By that stage command on a shiny new jet is probably not too far away, which makes a command on a 310 far less attractive, therefore none of the eligible F/O's are likely (and i dont blame them) to accept a command on that ageing piece of crap.
As many others have said. All this is a product of ill thought out knee jerk reactions to problem in other airlines.