We can begin by knowing what they share in common, storing data. Imagine data as boxes.

Brief definitions for a general idea:

var: used to store a box that will probably change later (mutable).
let: similar to var but a bit different (mutable).
const: used to store a box that won't change later (immutable).

Block, Function, and Global scopes:

We've to first know what's scope before knowing the difference between them.

scope: the part of the code for which a variable is defined.

Examples:

Global scope: anything outside of a function.
Block scope: anything inside curly brackets {}.
Function scope: anything in a function.

function b() {
  //Function scope
}

For now, you can think of each scope as its own room.

Difference between var and (let and const):

In Global scope: all of them can be accessed from other functions and blocks.

const a = 20
var b = 30

function c() {
   console.log(a, b) // 20, 30
}

In Function scope: var, let, and const can't be accessed from outside of the function.

function d() {
   let a = 10
   var b = 20
   const c = 30
}

console.log(a, b, c) // Error: a, b, and c are not defined

In Block scope: var can be accessed from outside of the block ({}), but let and const cannot, and that's the main difference.

{
   var a = 30
   let b = 2000
}

console.log(a, b) // 30, Error: b is not defined