in what respect,,,,

Whilst booking my holiday I was told to fill in an API form 3 months before I go to majorca.

I suppose it depends on what you mean by compulsory, most company's you book with now state that you should enter it on line or give it over the phone, or direct to your travel agent so many days before you travel.

It's the information they take off your documents at check in anyway, but I think they now like to have it in the system in advance to check against the documents, probably saves time at busy airports.

I would worry about a jobsworth check in clerk refusing me boarding if I didn't do it. From what i remember it's only you name, address, DOB, and passport details

If API is not provided in advance, it is taken at check in. Some countries require API to be provided at least 72 hours before travel.

What is the problem with providing it in advance?