I don't think there is such a thing as "laziness" in grammar. You can't just omit things in the middle of a sentence because you feel lazy and have a grammatically correct sentence.
The reason why floors 2 and 3 is used without an article is because in English we usually don't use definite articles in front of things that have numbers following them. That's a well-known fact. For example, when you're staying at a hotel, you would say this when talking about rooms:
I'm staying in room #35.
One way to think about this is that the number 35 uniquely identifies the room you're talking about and 35 (or 35th) is not being used as an adjective describing the room like in the example down below where you would need a definite article:
the 35th room
Again, just think of 35th as an adjective describing a specific room that you have in mind.