可是 however E.g 他想走，可是我不想走。( He wants to go, however I don't want to go. ....but maybe not bad idea go with him.) 但(是)but--- stronger tone E.g：他想走，但是我不想走。(He wants to go, but i'm definitely not going.) 即使even though E.g：即使他想走，我(也)不走。( Even he is going, i'm not going。)

但= But 可是= but / though 即使= even though

可是/ 但/ 但是 = but/ however 即使= although/ even if/even though generally, we use 即使...也...

每人都知道珍惜时间，可是为何很多人直到生命即将走到尽头才知道时间的宝贵。 鸡肠草和鹅肠草叶子十分相似，但是药性不同。 即使遇到再大的困难，人民解放军也会迎难而上决不畏惧！ 用可是、但、即使造了三个句子，也许为你对这三个词组的理解有所帮助 。

almost,they are equal but first two are same mean and there is a little difference in latest one. It emphasizes that you had conceded some facts,simutaneously,turn point