Correctly rejecting a null hypothesis

#1
Hi all, came here for a bit of advice. Got a bit confused - I think its a fairly simple question.

Goal
I want to determine whether Sb (Antimony) accumulation is greater in one central area relative to two nearby suburban areas (pre-existing policy implies that is the case).

H0: Sb accumulation in the central urban area is not significantly greater than Sb accumulation in suburban areas.

H1: Sb accumulation inside the central urban area is significantly greater than Sb accumulation in suburban areas.

I'm going to use one-way ANOVA with Tukey's multiple comparison test.

I was just wondering what happens if I find that only one suburban area has significantly greater accumulation. Am I able to reject to the null hypothesis? Is there a better way I should be approaching things?

Thanks