Some basic ideas for home security. You want layers of defense. You want the bad guys to have to come thru or over as many walls or barriers as possible. Since your on the 3rd floor in a locked building you have a bit more security to start with than most apartments I have lived in.
They have to get into the apartment building first (how well do you know your neighbors? get to know some, people that know you even a little are far more likely to help, even if it is simply noticing an odd person hanging around).
How solid is the door(s) to your apartment? Could you put holes in it real easy with a regular hammer? The stronger the door the better. Does your door open into apartment or out into the hall?
If the door opens in most healthy males can probably bust it open by kicking or body checking it. If it opens out then one has to pry it open, much harder without tools.
If it opens out that means the hinges are on the outside, and if the hinges are removed the door can be popped open pretty easy. To fix that you remove two screws per hinge (matching pair one on door and the one it lines up with on the frame) room the hinge and replace it with a nail. You want to leave the nail sticking up a bit so the head goes into the opposing hole (ie if you put nail in frame, the head will be in the screw hole on the door when the door is closed, that way even if they take hinge pin out they still can’t just pull door open).
Get some dumbbells, not little ones. Go to real sporting goods store that sells weights and benches and such. Get a matching pair of the heaviest cast dumbbell you can lift using two hands on one dumbbell (it should feel heavy to you to pick up). If your door opens in you can stick a towel under the bottom of the door to wedge it shut and then put dumbbells against the door also. Dumbbells that are big enough (I think 50 pounds smallest to work really good but 30’s and 40’s work fairly well) make VERY good tripping devices all by themselves. You can set them between exterior door and short line path to bedroom. The idea is if someone gets in they are slowed down, giving you time to go out window on fire ladder, or for neighbors to help or for cops to get there (call Fire Department, they are usually MUCH faster than LE), or whatever. Gives you time.
The other nice thing about dumbbells is that you can put a LOT of weight in one spot. And even if the get thru the door they still have to get thru the obstacle course. You can also use furniture and such to create a little room around the door. If you do that you can also you couple of broomsticks, rope, or chain to close the walkway when you go to sleep. Can also set motion sensor with light to blind anyone coming in the door. Might work.
You can also get this
http://www.foxlabs.com/Grenades.html
type of pepperspray if some starts trying to break thru your door (note pepperspray might make more sense for bedroom door, or at least store it ready to go there.) just stick tube under door.
Then you can flood the area outside the door with pepperspray.