Copy entries between tables

Say I have 3 tables with a structure similar to this:

| id | customer_id | col1        | col2        | col3        |
|----|-------------|-------------|-------------|-------------|
| 1  | C123        | some values | some values | some values |
| 2  | C234        | some values | some values | some values |
| 3  | C345        | some values | some values | some values |

Now I want to copy the rows from table 1 and table 2 to table 3 with a specific customer-id.

So in pseudo code something like:

COPY FROM tab1, tab2 TO tab3 WHERE customer-ID = C123