“应该”is equal to “should” " 一定" means “ certain” " 当然" means "sure" in my opinipn,当然 is the highest level in these and expresses you 100% fix the thing or matter. "应该" is the lowest level. Furtherly speaking,they have different moods. For example,I "当然"understand what you said。 I ”一定“guarantee I will accomplish the files this afternoon. I "应该"be free this Sunday.

应该 There are two meaning of this word.1. should 2. supposed to (verb)/ has already done Hmm, if you felt confused about these 3 words, i guess "应该" means "supposed to (verb)/ has already done". 我想他应该到了。 I think he has arrived.(He is supposed to have arrived yet) 他应该学中文 He should study Chinese. 他一定到了。 He must arrive. 他当然到了。 He arrives, of course. "当然"= of course. 当然和一定，not too much difference between them. Hope you get it:)