One difference is that you can use a complement with “en dessous” ou “sous”, whereas you can’t with “dessous”; thus, you can say this;

• le numéro de série est marqué en dessous de/sous la date de fabrication.

These two words are prépositions.

When you use “dessous”, which is not a preposition (so you can’t use nouns after it) but an adverb, you must specify in the context the thing under which is what you are talking about;

• La date de fabrication se trouve au dos du couvercle; le numéro de série se trouve dessous. The number is under the date.