1, dribs and dribs

study English in dribs and dribs 点点滴滴地学习英语

2, bit by bit

3, little by little

Bit by bit, inch by inch, little by little, a little at a time.

Step by step 也可以。

英语里也说“drop by drop"，但很少用，除非那“点点滴滴”的是液体或者可以被比喻为液体的东西：

Made to taste drop by drop more than the bitterness of death.