In my opinion this three has many differences but still one goal to have a successful marketing and to have a fund to raise their platform, ICO is the old marketing the Initial Coin Offering where they can invest on the dashboard or by just having a selfdrop, STO is Security Token Offering if I'm not mistaken there is a lot of legalities that is needed because it was terms of security they need to comply on SEC and last the most favorable one is the IEO or the Initial Exchange Offering this is the most popular for this year it as easily listed on the exchange after the IEO.