Though, "abajo" and "debajo" are used indistinctively by native speakers, "abajo" is related to motion, whereas "debajo" refers to a position. Pretty much the same difference in English with "on" and "onto". I can say "the water is on the table" (the place where the water is), and I can say "the water is being poured onto the table" (where the water is going to).
Examples:
El computador se fue abajo = the computer went down.
El computador está debajo del escritorio = the computer is under the desk.
As for "bajo", it is an adjective, (not an adverb like "debajo" and "abajo") meaning "short".
Example:
El niño ya tiene 8 años, pero es bajo para su edad = the kid is 8 years old, but he is short for his age.
I hope this helps.