It's a very simple risk equation. Balancing the risk if we keep him - that "this" is all he has, he doesn't develop, etc., vs the risk that if we sell now, we miss out on his potential and don't necessarily get duly rewarded for it. On the other hand, we get £Xm in the bank that we could spend (although with no guarantee that will work out), versus the risk that we have a Ribeiro/Brown situation, where we're offered a moderate fee by a bigger club, don't take it, but then never get that offer again.
I could write an equation if I could be bothered.
Me? I think £3m doesn't get you a lot these days, whereas we know what we have already. I'd be inclined to keep him another year, see how he goes.
But ultimately, the player himself will probably decide.