Can I have 2 or 3 scrap iron scarecrow face down on the field activating independently for each monster attac?

Yes you can, as long as they still remain unlimited, you can have up to 3 on the field. The only thing you have to remember is they can only be used once per turn, as they are set back on the field after the block, which is the same as setting a trap while its your turn (can't set jar of greed down then activate it right away, unless by card effect).