What charger only charges 3 batteries?
Actually several lol. Most of your cheap ones that come with rechargeable batteries are considered "dumb" chargers because they will only charge in pairs of two and do not charge the batteries based on voltage or amperage but on a set time frame. For instance the 15 minute quick chargers do not care whether a battery is fully charged or dead when when you put it on the charger, it is going to usually charge the battery at 1 amp for 15 minutes and then be done. This will lead to shortening of the battery's useful life, how long it will hold a charge, as well as not keep all the batteries balanced.
To get the most out of a set of rechargeable batteries a good charger is a must. Chargers from Nitecore, Xtar, and La-Crosse will all charger any number or combination of cells from 1-4. These chargers have a lot better charging parameters than the little chargers from Walmart and will monitor both the voltage and charge current required. Each slot is treated as a separate charger and the charger will do what is needed for that one battery. Below are links to three chargers worth looking into. Starting with cheapest and most basic going to most expensive and complex:
-Nitecore D4. Simple charger you put the battery in and wait for it to be done. I use mine quite oftem because its just so dang handy and does a great job. Will charge lithium/NIMH/NiCad batteries:
--Info
http://charger.nitecore.com/product/digicharger-d4
--Purchase
http://www.illumn.com/batteries-chargers-and-powerpax-carriers/chargers/nitecore-d4-digicharger.html
-La-Crosse BC-700. The Cadillac of battery chargers for AA/AAA rechargeables. This charger can act as a simple put the batteries in and let them charge or you can get more advanced with it and select individual charge currents for each slot or a different function. This is the charger I recommend if you only need to charge AA or AAA batteries and nothing else. I have one and wouldn't trade it for anything.
--Info
https://www.lacrossetechnology.com/bc700/
--Purchase
http://www.amazon.com/gp/aw/d/B000RSOV50/ref=mp_s_a_1_1?qid=1458823019&sr=8-1&pi=SY200_QL40&keywords=lacrosse+bc-700&dpPl=1&dpID=41gJ5-yLA2L&ref=plSrch
-Xtar VP4. This is the best charger available if you need to charge any size of NIMH or NiCad batteries such as AAA/AA/C/D or any size lithium battery. Mutual charging options available. This will be my next purchase and it will replace y other two.
--Info
http://www.xtarlight.com/05-chanpin/p-001-1.asp?styleid=322
--Purchase
http://www.illumn.com/batteries-chargers-and-powerpax-carriers/chargers/xtar-vp4-li-ion-charger.html
I also highly recommend Eneloop rechargeable AA and AAA batteries. They are THE best rechargeable batteries available in those sizes and outperform any other other brand. They hold a charge longer, have more charging cycles and do not take a memory like others.
I hope this didn't come off as insulting as I was only trying to provide more information about available chargers. Flashlights, batteries, and chargers are my main hobby so sometimes I get carried away.