In Modern Warfare 2 you don't choose three perks, instead you customize and select a perk package for yourself, which consists of two "Base Perks", one "Bonus Perk", and one "Ultimate Perk". You start the match with the "Base Perks" you selected, then after about four minutes into the match you unlock the "Bonus Perk", and eight minutes into the match you unlock your "Ultimate Perk".
Kills, assists or objectives let you unlock the perks faster. Some content creators have already tested this and according to JGOD each kill reduces the waiting time for the next perk by about 12.5 seconds.