Hi Dawn!
A subgroup of three may be right for you, five may be better, one may be correct also. Look at your process and determine the correct sample size and frequency. When we start a process, we take many samples in a short time, identify the assignable causes, remove or reduce them, start the process again, take less frequent samples, assess control, and so on, until the assignable causes are eliminated (or at least identified) and we know how long we can run until variation is expected to show up, thus samples are taken prior to this, for possible adjustment.
You can do this for present processes also, if needed.
I find that frequency has more to do with "control" than the sample size, except with multi-cavity tools.