What's the difference among the following?
- Isn't it the same radio that I lost last year?
- Isn't it the same radio as I lost last year?
- Isn't it the radio that I lost last year?
I'd appreciate your help.
First of all, the natural way of expressing this does not use "it." You are talking about a specific radio, so you would say "that," or if you are holding it in your hands, you would say "this."
Isn't that the same radio I lost last year?
Isn't this the same radio I lost last year?
Even better is a modification of your last option
Isn't this the radio I lost last year?
Isn't that the radio I lost last year?
You can insert the second "that"
Isn't this the radio that I lost last year?
To address your original question about "as" and "that," I would not use "as" without any other words in this context. You could expand it to
Isn't this the same radio as the one I lost last year?
But it's overly wordy in my opinion.
Both as and that can be introducers of a relative clause. They serve the same purpose. However, the word as has fallen out of use in this role; it was far more popular in the 18th and 19th centuries than it is in standard English today, though it does survive in regional dialects. Thus, using as today in that role could well have sociolect implications.
Your question is complicated by the fact that same is involved, and same...as is still current.
My car's the same color as his. standard/idiomatic
That's the same car as was stolen last year. non-standard