三个词都表示有可能做某事的时候.occassion一般指社交上从事某活动的合适时机.i will speak to him if the occassion arise.
opportunity、chance指有做某事所必需的实际环境.i hope you get a opportunity/chance to relax.
但是 chance还可以指可能性的程度或概率.如：：what are your chances to being promoted
而occasion还可以指某事发生的时刻.i met her on several occasinos recently.