Fare Pricing

I have just priced a flight from Alicante to LGW on 25th July.Quoted as €5.99 plus taxes for 1 pax.
When I request the same flight and timings for 3 pax the quote is for €49.99 plus taxes per pax.
I cant just book three separate single journeys as two of the pax are under 16 and they must travel on the same reservation as an adult.
Has anyone else come across this.I cant work out why this should be unless a computer is preset to offer a price when the remaining seats are down to a preset number, or Ryanair know that in School Holiday time that a group of three will probably be composed of 2 adults and 1 child and are maximising profits.

I quote myself from other topic at this phorum:

Yes it is possible, it's even expected. The company is giving some small amount of "cheap seat places" in aircraft, so if it'll be booked, then the offer is no longer active. Even more, if for example there is only 10 seats with this offer and you want to book 15 places with one reservation, the price for all of them will be higher. So you'll need to book 10 tickets first, and then more as you need.

This way I've lost Majorka january this year