I am sorry, but with a frequentist approach, you cannot tell how large the effect is in the population.
You can only "predict" (or conclude) that the effect is not zero, i.e. that some means differ in the population.
Of course, if your sample sizes are very large, then the sampling error is small and the parameters which
you found in the sample are roughly representative of the population. But with small samples, there is
large sampling error. You could look at the 95% confidence intervals of your three group means to get
an idea of how (un-)reliable the estimates are.
From your results I would maybe doubt that the analysis makes very much sense. Do you want to make
inferential statements based on group sizes of just n=7 or even n=5?
With kind regards
Karabiner