Any more information on the kid? Have you seen him play?
Agreed and this is just one potential reason why he should be looking for a shorter contract. Essentially, in my opinion at least, if he's good enough that we should be spending £3m on him in the first place it will most likely be in his best interest to get a shorter contract. Only if he's actually not that good, or he picks up a career threatening injury, or he completely stagnates or is a bad pro (knock on wood for all) would it benefit him to get a 5 year deal instead of a 3 year deal.
Even if he's not looking at us just as a stepping stone it would most likely be beneficial to have a shorter contract. Most likely his wages now, coming from Poland and not as someone targeted by the superclubs, will be relatively modest by PL standards. If he breaks in to the first team in 1-2 years his contract will be up for re-negotiation if his initial contract is 3 years and he'll be looking at a solid bump in pay. Even a year from now he might be in a strong negotiating position with just 2 years left even if he's only shown a handful of good performances or caught the eye with some individual brilliance even in a single game as 2 years left is about the last chance to avoid contract panic for the club.
With a 5 year contract on the other hand we could potentially keep him on that initial salary for his first 3 years and not panic. The club could also hand him a bump in pay (as seems likely with Bentaleb for example) and the player will be expected to sign an extension or perhaps add extension clauses to his contract, perhaps both. All of a sudden we have a contract lasting until 7 years from now after that first bump in pay instead of 5 years from now if his initial contract was just for 3 years.
Part of why Arsenal's wage budget is so high is that they've been handing out a lot of very player favourable contracts to highly rated and sought after young players I think. Then they end up having to bump their pay a couple of times by the time they're 24 to the point where the only way they can then further bump their pay is to leave or by Arsenal breaking their wage structure.