#1 Bitcoin - Bitcoin was created in 2008, it was the first ever cryptocurrency. Bitcoin is a digital currency based on a distributed ledger technology named Blockchain. Bitcoin eliminates the need for centralized intermediaries like banks, credit card companies, etc. When making electronic payments. If you want to pay 1 BTC to your friend, you can do it directly without having to involve your bank at all. Its aim is to create an alternative for flat currencies, like USD, GBP, JPY,etc.
#2 Ethereum - Etherreum was created in 2015 by Vitalik Buterin, it is actually more than a cryptocurrency. It is a Blockchain-based platform for developing decentralized apps and smart contracts. Ether is the native cryptocurrency used for all the transactions on Eutherium's Blockchain. What that means is that if you have an idea for a project/application that uses blockchain technology, you don't have to develope your own Blockchain, you can start building it on Eutheriums Blockchain.
#3 Ripple - Ripple is a very unique cryptocurrency as it is focused on solving problems that are related to just one industry - International Payment Transfers. It was founded in 2012 with the aim to make international transactions fast and also cheap. Of the total number of 100 Billion XRP that will ever exist, 50 billion of them are owned by Ripple labs (company that owns Ripple). This is a very different approach compared to most other cryptocurrencies where there is no central entity holding the majority of the coins owned.