Hi! Trios are generally the most tricky constellation to get right as they end up with a 2+1 outsider situation of some sort more often than not. Boars are best in personality based bonded pairs. If you want more, you have to go with sows.
If you specifically want a trio, look for a 2 sow and 1 one neutered boar combo. The two sows need to be friends for it to work, but you are generally working with piggy instincts rather than against them.
This guide here discusses all the pros and cons of various constellation in detail:
Boars, sows or mixed pairs; babies or adults?
Ideally you find guinea pigs at a good rescue where you can be guaranteed to adopt only guinea pigs that are guaranteed healthy, not pregnant and suitable (used to handling) for a home with children. We haven't got one that we can recommend warmly in your area, but the RSPCA Macclesfield has made an effort to improve their piggy section.
RSPCA Macclesfield - Rescue Rabbits, Guinea Pigs and others needing Homes
Please be aware that shop piggies have barely had any human contact and can be very nervous and wiggly.
Also be aware that guinea pigs living in a hutch need a place to be out of wind, frost and dampness for the winter and the hutch needs protection during heat waves. Guinea pigs die from heat stroke.
You and your parents can find a lot more information that your need to consider via this link here:
New Owners' " How To" Starter Kit