My rule on trading of draft picks is simple: I wouldn't allow it.
If you feel like you have to allow it, I would put in the same restrictions for draft picks as you do for players. Having someone else's draft pick counts towards your 3-player maximum. If you don't do this, you would short-circuit the whole point in having a keeper limit. If some guy punts on a season, has 3 decent keepers, and then trades all the rest of his guys for 6 extra draft picks, it's not going to be a very fun league for everyone else the next year. Of course, once you put in that rule, people will say, "Well, then what's the point of trading draft picks" and they'll probably be right, which is why I just wouldn't get into it.