Sorry 'bout your lil fish. I loved my beta.
That said, when we moved out of the country, we had to leave him with my brother and my then 5-year-old nephew 'adopted' him. Well, Mr Fish got sick after a little while and, uh, my brother and sister-in-law had to "take him to the doctor", if you follow me.
The next day, they took my nephew to the pet store - which is where the fish doctors are, of course - and got him to point to which one was Mr. Fish on the shelf. And then they took Mr. Fish home again, and all was well.
I always thought that was a clever way of dealing with it for a little kid like that. No worries that he'll say Mr. Fish was the wrong color or something, I guess? Dunno if it would work out for a 3 year old, but it's a thought anyway. I remember having a number of fish funerals when I was little. I guess that works too (although 30 years on, all I remember about my goldfish is they dropped like flies...).
Good luck!