"Open" apps do not consume battery unless you are actively using the app (the app is displayed on the screen.) Some -processes- use battery, but only when they are actually doing something like synching up with your e-mail (so a few seconds use every 15 minutes, for example).
Android manages memory, applications and processes on its own and you don't need to do anything special. If you can't see an app on the screen, the app is basically hibernating. If Android needs the memory for something else, Android will auto close the least used app(s) until it has the memory it needs.
A battery manager may help, but task killers do not have the benefits that most people believe they do and won't help you much. (Google the phrase "don't need a task killer in Android" and several articles will come up if you want to learn why task killers are not a great idea.)
Three hours for battery drain is very goofy. I have WiFi and GPS on all the time, make phone calls, surf the web (etc.) and my battery lasts all day. I just plug it in every night. I've never used the phone so much in one day that I've used up the entire battery charge.
Hit the menu button, then Settings -> About Phone -> Battery Use to see what processes are eating up the most battery. I'm not sure what is happening with your phone, but three hours is abnormally quick for the battery to get used up.