1. There is pieces of a soap bar on the table.

Question

2. There is a soap bar on the table.

3. Thre is pieces of soap bar on the table.

(Which ones are grammatical? Do you use some other expressions?)

Answer 1

Only #2 is grammatical.

Pieces is plural... "is" is singular.

There is a bar of soap by the sink.
This is a common usage.