counsellor - a person who has been trained to advise people with problems, especially personal problems 和 adviser - a person who gives advice, especially sb who knows a lot about a particular subject，基本上可以互用，而counsellor更加职业化，例如学校里给出学生学习或者职业生涯建议的学业顾问，还可以表示律师；adviser更普遍化，你的朋友给你一些购物的建议，那么他可以叫你的adviser，却不能叫counsellor。

consultant - a person who knows a lot about a particular subject and is employed to give advice about it to other people，是指给出专业，高水准建议的顾问，例如顾问医师。它更贴近与中文里“顾问”的含义，因为我们所说的顾问通常都有“专家”的含义。