You can series them and use the charger that Tom mentioned, BUT don't shoot yourself in the foot by not charging each one of those batteries individually before putting them all together!
A lot of diy'ers just slap batteries with different SOC's together, and cross their fingers with the higher overall voltage charger. Either that, or they assume that new batteries don't need a good initial charge - by themselves first!
Battery Tender makes one suitable for this purpose - the 1.25A AGM 12v model. There are plenty of others, but this is the a common reference.
Once you have charged each one individually, and then LATER connected them up in their final configuration, watch over them during charge and make sure they don't vary more than 0.1V from each other at the end of charge. This is also a PM procedure. They should not vary more than 0.1V during rest - if they do, that may be an indication that one or more batteries was manufactured poorly, or perhaps your own wiring infrastructure is flaky. (loose clamps, corrosion etc).
Even though those batteries are low-cost, this is the same thing you'd do with a much more expensive bank.
If you really want to get into it from a "learner bank" standpoint, you may want to pick up an inexpensive infrared temp/laser pointer at your local hardware store. As you charge, point the temp laser all over your cells, and especially along your infrastructure wiring. Overall accuracy isn't as important at identifiying immediate "hot spots" that allow you to correct things (like loose clamps) before they become an issue.
Sounds like overkill, but they are inexpensive enough, and still quite useful if you decide to go with a larger current / capacity format.
