BIG HUGS
I am so very sorry that you have lost Teddy. What a horrible shock for you.
Strokes (more likely in your case) and heart attacks can sadly happen at any age out of the blue in seemingly healthy piggies. They are not foreseeable or preventable and they can happen to any owner. I have lost several of my own piggies this way.
Please try to take consolation that your boy cannot have suffered for long. As natural deaths go, this is one of the kindest because it is so fast and it hits out of the blue without any illness.
Here is our grieving guide for owners and families, which you may find helpful:
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children
3 years is the prime of life. The average life span is about 4-7 years and sometimes even longer but you can never choose or predict how long you have your piggies. As long as you give them what they want from life, many happy todays in good care you are not failing them. Guinea pigs don't have a concept for an average life span, they live in the moment. It matters less how long they live than how well they have lived.
Please leave the cage untouched to allow Teddy's brother to grieve and to be able to snuggle into his scent while it slowly fades. It doesn't matter for once if the cage is getting grotty because the comfort factor is more important. You can clean the cage once he starts picking up normal life, if without his sparkle again.
Unless the companion stops eating and drinking altogether (and even a little syringe feed cannot kickstart the appetite again) and is giving up on life (i.e. acute pining) or unless he can obviously not cope with being on his own at, you usually have between 1-4 weeks (in the case of neutering or adopting a special piggy even longer) time to find a new friend, quarantine them if needed or allow them to settle in and get their bearings next to Teddy's brother for a few days before any bonding. Rescue dating - if possible - would be another option.
Here is our very helpful guide on what you can for newly bereaved piggies in the immediate to longer term. We are always happy to answer any questions in our specially Care sections so you are more likely to get knowledgeable answers quickly:
Looking After a Bereaved Guinea Pig
This will give you time to get your own bearings. If you struggle with aspects of your own grieving process or with Teddy's brother, you are welcome to open an ongoing support thread in our new End of Life & Bereavement Support Corner in the Care section part of the forum. What we can provide is our community support and the understanding and experience from other owners who have gone through similar experiences. The corner provides a friendly place to ask any questions and concerns you have along the way. Since we are not part of social media, we can let ongoing personalised support threads run for as long as possible.
Here is the direct link to the section:
End of Life and Bereavement Support Corner