In a one-way ANOVA, what does it mean to reject the statement in the null hypothesis if three treatment groups are being compared?

This means we reject that all the three groups have same means.

Step-by-step explanation:

given that in a one-way ANOVA, we reject the statement in the null hypothesis if three treatment groups are being compared

In anova one way we compare the means of more than two groups.

The hypotheses are set up as

H0: Means of all three groups are equal against

Ha: atleast two means are different

When we find out F statistic by dividing SST by MST

we get p value.

Whenever p is less than significant level, we reject null hypothesis

This means we reject that all the three groups have same means.

Then we can go further to check up which pair is different by doing Tukey test or pairwise t test.