Resource guarding can be managed and in many cases resolved but you need active management in place to make sure bites do not happen - get in touch with a reward based trainer and see if they can do a 1-1 visit to help you determine if you are able to resolve this and keep things safe - especially with children - was the 5 year old yours?
My girl came to me at 17 months as her first owners were not able to cope with her resource guarding - she is now pretty much a reformed character but I do not have children living with me and can keep her safe from them. She likes children but a dog who is guarding will give warnings which need to be heeded and small children are not good at that so if you have children you need to ensure their safety too and if you can not do that it is worth speaking to rehoming organisations to see if they can help. In the UK we have the Doodle Trust who will take on guarders but for bites it would probably depend on the frequency and severity of the bite.
The 3 people were people who he wouldn't have had much interactions with before. The first time there was a lot of people around and he growled at sometime who went to pet him, they pulled back but then another person tried and he bit them. The second time he was tied up, which we rarely do but we were visiting parents for a family party and could not have him roaming. My brother went over to him and he bit him on the leg. Not sure what /if my brother did anything to provoke. The 3rd time we were visiting friends and he was playing in their garden, their 5 year old gave him a stick to play with. After a while he went to get the stick back from our dog who had left the stick near. He bit him on the arm. Each bite drew blood.
OK then sounds like all were things that with the benefit of hindsight could be avoided so with careful management and help from a trainer I can't see why he would need putting to sleep. The important thing is to make sure he is never put in a position to be able to bite again - the first two bites sound like he was stressed and told the people this but was not listened to and so escalated. If things are busy for your boy you would be far better to shut him away so he does not have to deal with it.
Guarding a stick is slightly different but if he was my dog I would never be putting in him a position like this again, especially with children.
Hope the little one is not too badly hurt - are the owners demanding put to sleep?