Yes, "a small, fancy bike" can be expressed correctly as "a small and fancy bike;" however, the commas are not necessary and the preferred phrase would be "a small fancy bike."
Further, "a good, earnest, diligent student" can be expressed correctly as "a good, earnest and diligent student. Please note that, were I your teacher, I would suggest omitting two of the adjectives and rewriting the phrase as "a diligent student." Together, "good," "earnest" and "diligent" are redundant as any one of the adjectives encompasses the remaining two. I prefer "diligent student."
In additiona, a writer must consider comma usage in relationship to the type of writing he/she is doing. Legal and/or business writing and creative writing, for instance, require different punctuation, especially as to commas. You may wish to consult a resource dealing with AP and Chicago styles.
Originally Posted by
enydia
Welcome!
Is a small, fancy bike same as a small and fancy bike? Yes it is, but you don't need a comma between "small" and "fancy". Say 'a small fancy bike'.
Is a good, earnest, diligent student same as a good, earnest and diligent student or a good and earnest and diligent student? Yes, and again we don't put commas between adjectives. Also "good" is probably not needed, as it sounds redundant. Say 'a (good) earnest diligent student'.
Thanks