1) Meaning: to carry on doing something but in an even more determined way.
I really need to D________ down on my Spanish studies if I want to be fluent by our trip in July.

2) Meaning: to visit someone or something for a short time.
I'll L________ in on mum on the way into town to see if she needs any shopping.

3) Meaning: to take strong action to stop or limit an activity that is harmful or unwanted.
The teacher is C________ down on pupils being late to class by contacting the parents concerned.