Many people use these words interchangeably; there is really very little difference between them. In everyday spoken English, the most common word is goal. Aim and objective are usually used in more formal writing.
ne small difference is that an objective is more specific than a goal, for example:
Our goal is to improve health care for children.
Our objective is to provide 10,000 children with vaccines.
However, in casual conversation, most people would use goal for both general and specific things:
My goal is to lose weight.
My goal is to lose 20 pounds by the summer.
The words goal and objective are nouns, and the word aim can be a noun or a verb:
The aim of this project is to increase our students' motivation.
aim = noun
We're aiming to increase our students' motivation. aim = verb