Subgroup analysis I have a total of 3 studies in a meta-analysis. Using a group of 1 and then a group of 2 for subgroup analysis gives me the results for group 2 but for group 1 it says heterogeneity can't be calculated. However, between the groups, it does give me the result. Does this mean I can use the overall subgroup results?

I assume you are using the measure $I^2$ to assess heterogeneity. The point of measuring heterogeneity in meta-analysis is to assess if the proportion of difference in the estimates between the studies is due actual differences in the associations rather than sampling error.  This measure is really not useful if you have too few studies, such as 3. Maybe it would be more insightful to just make a descriptive comparison between the studies! For instance, you could compare the sample characteristics and the difference in the model specifications (if there are any).
To answer your specific question: if in one group you only have one study there is no heterogeneity to calculate. To make an analogy, it's like a dataset where you only have one observation, you can't calculate the variance! And in the case you have two observations, it’s like computing the variance among them; you can still do it, but it might be more useful to try a different strategy.
