Hola Emma

There's a bit of subjectivity here; generally, if you start the sentence with the verb (Me gusta/n...) if you have two subjects both in singular, despite having two subjects, it's more common to use the verb in singular:

Me gusta el chocolate y la golosina. 

You could use the plural gustan too but the singular is generally preferred. It is as if we omitted the verb before the second element: Me gusta el chocolate y [me gusta] la golosina.

In your 2 and 3 sentences, you have plurals in the subject, so there I'd use the verb in plural:

Me gustan el chocolate y las golosinas.

Me gustan las golosinas y el chocolate. 

If you were talking about liking "activities" using infinitives, even when you talk about more than one activity, you'll still see the verb in singular:

Me gusta cocinar y planchar.

Me gusta ver la tele y jugar a videojuegos.

I hope it clarifies it.

Saludos