I know how you feel, having your other two will fill the void, but there will always be a space left.
I always remember a line of a poem I once read 'a part of me went with you, a part of you still stays'.
They all take a part of us when they leave, but I know that a part of them still stays in my heart. No-one can take that away from me.
I think that my friends are still with me, I have even felt like I have seen them sometimes. I am sure that although they are not with me physically their spirit still runs beside me, coming along on the walks, protecting me. I believe they go to a different 'plane', they don't feel any pain and are back to full health and happiness.
Don't blame yourself, you were not to know the side effects of the drug, the vet should have warned you, if you follow the link that Neil gave on that answer it gives in detail what the vet should do before prescribing the drug. From what I have read once the drug is in the system there is very little that can be done if they have a reaction to it.
You did what you thought was right, you are not a vet and you trusted the people with the qualifications, there is nothing wrong in that.
Like I say - take one day at a time, eventually the raw pain will become a dull ache. He will always live on in your heart.
Take care.