Hi!
It would be great if you considered going reputable rescue; that will save you from the usual pitfalls for the unwary, i.e. acquiring ill, missexed, pregnant or badly bonded piggies. Most good rescues also offer a bonding service, so would do the intros for you under expert supervision - that minimises your risk further and you can be sure that your piggies will get on when you take them home. A good rescue will also support you with advice during the life of their rehomed piggies.
Some rescues have already neutered boars. It is worth asking around or going a bit further afield for, as sadly post op complications are not at all uncommon, and they can occur weeks after the operation. You will need to ask around for very experienced vet if you want to minimise the risks for yourself. A neutered boar will also need a full 6 week post op wait to be 100% safe - I have an over 5 week post op surprise baby to make that particular point! Boars also need to be a certain minimal age/development before they can be neutered.
You can only have one neutered boar with any number of sows - one of my own boars (who nearly died from post op complications) is now happily living with 11 girls.
Three guinea pigs need a minimum of 2x5 ft accommodation. If you want to keep them outside, you will need to have some winter provision to keep them out of the worst of the cold and damp weather.
Here is a link to a map of recommended guinea pig rescues: https://maps.google.co.uk/maps/ms?msid=209994852075231951564.0004b8fd9391b4257d8eb&msa=0