What is the format? Can you keep any three players for three years? Is there a penalty for keeping (lose round drafted each year kept, salary cap, each year kept the round goes up, etc)? Starting requirements? Roster size? How many teams in the league?
Some of that information will be helpful to know but in general with that few keepers I treat it just like a redraft. I wouldn't worry about forcing young players strictly on age. I go for the best value at the time I am picking. If I have two guys evaluated the exactly the same I might give a nod to the younger guy but it's not the end all be all. Depending on who you are playing with you may be able to get great value with guys that are still very productive but are a bit older. Many times new owners will go overboard on the "must take young guys" approach and forget about the current year. Bottom line for me is with only 3 keepers I don't change anything from a re-draft and I try and take it one year at a time.