Actually if you're smart you probably won't and I'll tell you why...
Here is a list of the killstreaks:
3 Kills:
UAV - Shows enemies of the mini-map.
4 Kills:
Care-Package - Airdrop a random killstreak or ammo.
Counter-UAV - Temporary disables enemy radar.
5 Kills:
Sentry Gun - Airdrop a place-able Sentry Gun.
Predator Missile - Remote control missile.
6 Kills:
Precision Airstrike - Call in a directional airstrike
7 Kills:
Harrier Strike - Airstrike with a hovering harrier.
Attack Helicopter - Call in a support helicopter.
8 Kills:
Emergency Airdrop - Airdrop 4 random killstreaks or ammo.
9 Kills:
Pave Low - Heavily armored assault helicopter.
Stealth Bomber - Airstrike undetectable on enemy maps.
11 Kills:
Chopper Gunner - Be the gunner of an attack helicopter.
AC-130 - Be the gunner of an AC-130.
15 Kills:
EMP - Temporarily disables enemy electronics.
25 Kills:
Tactical Nuke - End the game with a bang.
You've got 15 possible killstreak bonuses.  Now you have to pick 3 of those to use.  So, are you going to pick 2 that you get to use and save the 3rd for the 1 in 500 games that you hit 25 kill streaks?  Probably not.  You're going to pick killstreak bonuses that you can use often.
The 15 streak EMP is achieveable (I hit 15 pretty often), but to stretch that another 66% longer isn't worth it to me.
I'd probably pick a UAV, Airstrike, and AC-130 or something like that.  Something you're going to actually use.