Mentors tell you what they did, advisors tell you what to do, coaches help you find the answer yourself. How founders use each, and where each goes wrong.

Founders collect all three, usually without deciding which is which. At the highest level:
Mentors are founders or operators a few steps ahead of you. A Series A founder might informally engage a Series C founder to talk through the problems common to the stage. Good mentors are role models and confidants. They help you see round corners and offer solutions drawn from their own experience. Two things to watch.
Advisors are subject-matter experts. A Series B founder might formally engage a multi-time CRO from their industry to help scale the sales team. Most startups build a bench of advisors over time, covering the areas where they lack experience. Two things to watch.
Coaches are usually former founders or operators who now spend most of their time helping founders grow. A coach might work on management and leadership by asking the questions that help a founder see their situation clearly and find solutions they hadn't considered. The work is aimed at root causes rather than symptoms. Two things to watch.
Each has its place. The best approach is usually to have all three and know when to lean on which.