1. Oh, it¡¯s on aisle 6, next to the coffee.

2. Oh, it¡¯s in aisle 6, next to the coffee.

3. Where can I find a dark chocolate bar? It's in Aisle 3.

4. Where can I find a dark chocolate bar? It's on Aisle 3.

(Do we have to use 'in' or 'on' before 'Aisle'? Are both OK? Which one is common?)

We use "in" before aisle. 2 and 3 are correct.

1. Ms. brown just had three girls.

2. Ms. brown has just given birth to three girls.

3. Ms. brown had three girls just now.

(Are they all the same and grammatical?)

Yes.

We assume that Ms. Brown just had triplets.

Both "in" and "on" can be used before "aisle" to indicate the location of an item. However, the more common usage is "in" when specifying a specific aisle.

For example, in sentences 1 and 3, "in" is used before "aisle" to state that an item can be found inside a certain aisle. This is the more common way to express this.

Sentence 2 uses "on" before "aisle," which is also grammatically correct. However, it is less commonly used when specifying the location of an item in a store.

Sentence 4 also uses "on," which is grammatically correct but less common than using "in."