Recently a flight I took had a very long delay (over three hours) and I think I could be entitled to a refund.
I am not sure this answers, but it may help:
The websites you mention in particular can't tell if they help or not, but I know some really work. Of course it's not easy to get a refund and the process is a bit "Kafka style". The companies won't make it easy. There are no guarantees that you get the money and even if you get something you may have to settle for credit in the company (to spend on flights) or a percentage of what in theory you should get back.
There are basically 2 types of websites/service:
they follow a workflow, based in your case, and give you the forms or letter templates, with some extra information about deadlines for the company and for you, and you need to send them yourself. Since a lot of information is currently in the Internet they can even go as far as checking if the flight was really delayed and based on that give an estimate and start the process. You follow the steps and with each step you answer their questions (Eg.: did the company answer on time, was it positive, was it negative, etc.). Based on that the workflow gives you the next step and form to send. They provide addresses and everything based on the European country where that happened. The process is automatic and the goal is to guide you through the process, that can be long. They will probably charge a fixed value.
They act as similar to 1. but they send the forms for you. In some cases they even give legal support as representatives (I assume they have lawyer firms behind them). There may be some forms that you need to sign and send yourself. Since they have more involved they will usually charge a commission from what you get back, and a fixed value to start the process like 1.
I have a friend that started 1. Back than I remember him telling me about 2.
I have not personally used these websites, but it is possible that these are legitimate websites.
In the EU, there are strong air passenger protections. From that website:
In addition, if you are denied boarding, your flight is cancelled or arrives more than 3 hours late on arrival at the final destination stated on your ticket, you may be entitled to compensation of €250 - 600, depending on the distance of the flight.
Naturally, getting payment for such a claim is made exceedingly difficult by the airline: they will repeatedly refuse to pay legitimate claims. A friend was flying back to the US from Spain and had a flight that was delayed for 24 hours for non-weather reasons and registered a claim. He made repeated requests for payment and followed up with both Spanish and EU regulatory boards regarding the matter, and the airline still refused to pay. Eventually, his hounding caused the airline to offer him compensation, but he held out for the full 600 euro he was entitled to, and received it, but it took more than six months of back-and-forth with the airline.
I was booked on the same flight with my friend, but did not follow up with the airline, as the amount of paperwork and repeated requests required was more effort than I cared to put in. This is where these websites would presumably step in, and handle the negotiation with the airline on your behalf, in exchange for part of the proceeds.