好像 is for conversation etc, 好似 is for classical and poetic forms. Both mean "looks like" or being similar 一样 = the same, is equivalent to,

好似 = 好像, however 好似is more formal than 好像 一样 means "the same as", you could say something like: 那朵云好像和人脸一样。

yes, i agree with Lashay. Besides, 一样means more silimar than "好像"or"好似"，it even means no difference. Then, 一样 and 好像 are offen used in the end of the sentence while 好似 is only used in the middle of sentence. For example: 那两只小猫长得"一样"。( the two cats looks the same ) 两只小猫长得好像。（the cat look similar with the other one） Finally: " 好似 " means looks like. For example: 他好似一个猴子。（he looks like a monkey） when "好像" is used in the middle of the sentence, it means "seems" . For example: 他好像很生气。（It seems that he is very angry）

Personally, ”好似“多用于书面语（which is often used in written language），“好像”多用于口语(spoken language )。They both mean "similar".

I'm not totally sure if there is a difference between 好似 and 好像 (which both seem to mean, similar or to be like), but 一样 is different from the other two because it means "the same." So if you want to say "they are the same," then you might say 他们一样.