First thing, define a type or interface for your object, it will make things much more readable:
type Product = { productId: number; price: number; discount: number };
You used a tuple of size one instead of array, it should look like this:
let myarray: Product[];
let priceListMap : Map<number, Product[]> = new Map<number, Product[]>();
So now this works fine:
myarray.push({productId : 1 , price : 100 , discount : 10});
myarray.push({productId : 2 , price : 200 , discount : 20});
myarray.push({productId : 3 , price : 300 , discount : 30});
priceListMap.set(1 , this.myarray);
myarray = null;