Thank you for using the Jiskha Homework Help Forum. The "would" is usually the condtional = je mangerais (for regular verbs it is the same stem as the future, in this case, the entire infinitive plus the endings of the imparfait (-ais, -ais, -ait, -ions, -iez, -aient.))
I should eat is the verb devoir (to ought to, to "should" = Je dois
I could eat is with the verb pouvoir (to be able to ) in the imperfect, the conditional, or even the passť composť. Here you need to decide if he "could" and therefore, he "did." = passť compose OR he "could" in the sense "he was able to" but you never know if he actually "did" = l'imparfait OR in a "si clause" like "he could if he wanted to..." = conditional plus l'imparfait in the "si" clause.
I might eat = conditional (and there is probably a "si" or "if" clause coming as well. (pourrais)
Well, this is NOT Level I or II and possibly not even III!